# onlinecalculator.me

> Free, fast, mobile-first calculators for everyday number questions. Every calculator runs entirely in the browser — no server calls or accounts. Inputs stay on the user's device.

## What this site is

onlinecalculator.me is a collection of focused, single-purpose calculators covering personal finance, health metrics, math, unit conversion, date arithmetic, construction, and everyday needs. The design is intentionally minimal — one question per page, answered immediately.

- **175 calculators** live across 7 categories
- All math runs client-side in tested TypeScript
- Each formula has ≥10 unit tests with citations to authoritative sources (IRS, NIST, WHO, textbooks)
- Lightweight static pages: a typical calculator page is ~13 KB gzipped HTML plus ~10 KB gzipped JS — no frameworks at runtime, search index loads on demand
- WCAG 2.2 AA accessibility: keyboard navigable, aria-live results, screen-reader friendly
- Installable as a PWA (works offline after first load)
- Share-with-values links let users share their exact inputs via URL query params

## What each calculator page contains

Every calculator page includes:

- An interactive widget (runs entirely in the browser)
- A step-by-step how-to-use guide
- The formula with all variables defined
- A worked example with real numbers
- A FAQ section (3–10 questions covering the formula, edge cases, and assumptions)
- Related calculators with one-line blurbs

Structured data on every calculator page: `SoftwareApplication`, `FAQPage`, `BreadcrumbList`, `HowTo`.
Category pages include `CollectionPage` + `ItemList`. Homepage includes `WebSite` + `Organization`.

## Formulas and data sources

All formulas are sourced from authoritative references. Key sources:

- **Mortgage / loan**: standard annuity formula M = P·r(1+r)^n / ((1+r)^n − 1)
- **Compound interest**: A = P(1 + r/n)^(nt) per standard finance textbooks
- **US income tax**: IRS Rev. Proc. 2024-40 (tax year 2025 brackets and standard deductions)
- **FICA (2025)**: IRS Publication 15 + SSA COLA — 6.2% Social Security up to $176,100; 1.45% Medicare
- **Long-term capital gains (2025)**: IRS 0/15/20% bracket thresholds, stacked above ordinary income
- **State income tax**: Tax Foundation 2025 state rates (flat-rate approximation per state)
- **BMI**: WHO formula kg/m²; CDC weight classifications
- **BMR**: Mifflin-St Jeor (1990), Harris-Benedict (revised Roza-Shizgal 1984), Katch-McArdle
- **Pregnancy due date**: Naegele's rule (LMP + 280 days)
- **Retirement**: Bengen 1994 safe withdrawal rate (4% rule), Journal of Financial Planning
- **BRRRR**: cash-on-cash = annual cash flow / cash left in deal; all-in = purchase + rehab + holding
- **Unit conversions**: NIST exact conversion factors
- **Auto loan**: same annuity formula as mortgage; sales tax applied to (price − down − trade-in)

Monetary calculations default to USD. Rates are annual percentages unless stated otherwise.
All calculations run in the browser — no data leaves the user's device.

## Who should use which calculator

- **Buying a home** → Mortgage calculator (monthly payment, total interest, amortization, LTV-gated PMI)
- **Investing in rentals** → BRRRR calculator (buy-rehab-rent-refinance loop; cash-on-cash return)
- **Buying a car** → Auto Loan calculator (includes trade-in, sales tax, dealer fees)
- **Saving for retirement** → Retirement calculator (nest egg projection, 4% rule income)
- **Filing taxes** → 2025-2026 Income Tax calculator (US federal brackets, opt-in FICA, state, capital gains)
- **Paying off debt** → Loan calculator (personal, student, any fixed-rate loan)
- **Checking a healthy weight** → BMI calculator (CDC classification, healthy range)
- **Tracking calories** → Calorie or TDEE calculator (Mifflin-St Jeor, macro breakdown)
- **Due date** → Pregnancy calculator (Naegele's rule, gestational week, trimester)
- **Splitting a restaurant bill** → Tip calculator (per-person amount, any tip %)
- **Converting temperatures** → Celsius/Fahrenheit calculator (°C, °F, K)
- **Converting weight** → Pounds/Kilograms calculator (lb, kg, oz, g, stone, and more)
- **Measuring a room** → Square Footage calculator (rectangle, circle, triangle; ft/in/m/yd)
- **Calculating days between dates** → Date Difference calculator (days, weeks, business days)
- **Finding someone's age** → Age calculator (exact years/months/days, next birthday)
- **Compounding savings** → Compound Interest calculator (any frequency, regular contributions)
- **Fractions homework** → Fraction calculator (add, subtract, multiply, divide; auto-simplify)
- **Grade point average** → GPA calculator (term + cumulative, letter grades)
- **Percentage problems** → Percentage calculator (5 common percentage problem types)
- **Scientific math** → Scientific calculator (trig, log, exp, factorial, memory, keyboard)

## Calculators

### Finance

- [2025-2026 Income Tax Calculator](https://onlinecalculator.me/finance/income-tax/) — Estimate your US federal income tax for the 2025-2026 filing season — official 2025 brackets, standard deduction, and an opt-in toggle for Social Security and Medicare (FICA).
- [401(k) Calculator](https://onlinecalculator.me/finance/401k/) — Project your 401(k) balance at retirement with employer match, annual return, and inflation adjustment. Includes a year-by-year growth table.
- [APR Calculator](https://onlinecalculator.me/finance/apr/) — Convert a nominal interest rate plus lender fees into the true Annual Percentage Rate (APR) so you can compare loan offers on an equal footing.
- [APY Calculator](https://onlinecalculator.me/finance/apy/) — Convert a nominal interest rate to Annual Percentage Yield (APY) for any compounding frequency — daily, weekly, monthly, quarterly, semiannual, annual, or continuous.
- [Auto Loan Calculator](https://onlinecalculator.me/finance/auto-loan/) — Calculate the monthly car payment, total interest, and full amortization schedule for any auto loan, including down payment, trade-in, sales tax, and fees.
- [Bond Yield Calculator](https://onlinecalculator.me/finance/bond-yield/) — Calculate current yield, yield to maturity (YTM), and Macaulay duration for any fixed-income bond. Enter the face value, coupon rate, payment frequency, current price, and time to maturity.
- [BRRRR Calculator](https://onlinecalculator.me/finance/brrrr/) — Analyze a Buy-Rehab-Rent-Refinance-Repeat real estate deal. Enter purchase, rehab, ARV, rent, and refi terms to see cash left in deal, cash flow, and cash-on-cash return.
- [CD Calculator](https://onlinecalculator.me/finance/cd/) — Calculate the final balance and interest earned on a certificate of deposit (CD) for any principal, APY, and term length.
- [Compound Interest Calculator](https://onlinecalculator.me/finance/compound-interest/) — See how interest compounds on a lump sum or regular contributions over time, with a year-by-year breakdown and support for any compounding frequency.
- [Credit Card Payoff Calculator](https://onlinecalculator.me/finance/credit-card-payoff/) — Enter your balance, APR, and monthly payment to see how long it will take to pay off your credit card and how much interest you will pay in total.
- [Currency Converter](https://onlinecalculator.me/finance/currency-converter/) — Convert between 20 major world currencies using approximate exchange rates. Includes a full conversion table for quick comparison.
- [Debt Snowball Calculator](https://onlinecalculator.me/finance/debt-snowball/) — Enter all your debts with balances, minimum payments, and APRs. Choose snowball (smallest balance first) or avalanche (highest rate first) and see exactly how long payoff takes and how much interest you save with extra monthly payments.
- [Discount Calculator](https://onlinecalculator.me/finance/discount/) — Find the final price and amount saved for any percent-off or dollar-off discount.
- [Home Affordability Calculator](https://onlinecalculator.me/finance/home-affordability/) — Find the maximum home price you can qualify for using the standard 28% front-end and 43% back-end debt-to-income rules, accounting for your income, existing debts, down payment, and local costs.
- [Hourly Wage Calculator](https://onlinecalculator.me/finance/payroll-hourly/) — Convert an hourly rate to weekly, biweekly, semi-monthly, monthly, and annual salary. Includes overtime pay at any multiplier and threshold.
- [Inflation Calculator](https://onlinecalculator.me/finance/inflation/) — Find out what a dollar amount from today is worth in the future, or what a future amount is worth in today's dollars, using any annual inflation rate.
- [Investment Return Calculator](https://onlinecalculator.me/finance/investment-return/) — Calculate the compound annual growth rate (CAGR) of any investment, or project a future value at a given annual rate. Includes total return, total gain, and the Rule of 72 doubling time.
- [Loan Calculator](https://onlinecalculator.me/finance/loan/) — Calculate the monthly payment, total interest, and payoff time for any fixed-rate installment loan — personal, student, or auto — with optional extra payments.
- [Markup and Margin Calculator](https://onlinecalculator.me/finance/markup/) — Calculate selling price, profit, markup percentage, and margin percentage from any cost. Switch between markup mode (profit as % of cost) and margin mode (profit as % of price).
- [Mortgage Calculator](https://onlinecalculator.me/finance/mortgage/) — Work out the monthly payment, total interest, and full amortization schedule for a fixed-rate mortgage, with optional extra payments, property tax, insurance, HOA, and PMI.
- [Paycheck Calculator](https://onlinecalculator.me/finance/paycheck/) — Calculate your take-home pay after federal income tax, Social Security, Medicare, and state income tax for any pay period. Uses 2024 IRS tax brackets and official FICA rates.
- [Refinance Calculator](https://onlinecalculator.me/finance/refinance/) — Find out if refinancing makes sense — compare your current loan to a new one, see monthly savings, and calculate how long until the closing costs pay for themselves.
- [Rent vs Buy Calculator](https://onlinecalculator.me/finance/rent-vs-buy/) — Compare the true long-term cost of renting versus buying a home, factoring in mortgage payments, equity buildup, appreciation, maintenance, and the opportunity cost of your down payment.
- [Retirement Calculator](https://onlinecalculator.me/finance/retirement/) — Project your retirement nest egg with inflation adjustment and the 4% safe withdrawal rate. Shows nominal and real portfolio value, annual and monthly income, and a year-by-year schedule.
- [Roth IRA Calculator](https://onlinecalculator.me/finance/roth-ira/) — Project your Roth IRA balance at retirement and compare it to a traditional IRA after taxes. See how tax-free growth compounds over time and calculate your Roth advantage.
- [Sales Tax Calculator](https://onlinecalculator.me/finance/sales-tax/) — Add sales tax to any price and see the final total. Supports optional pre-tax discounts.
- [Savings Goal Calculator](https://onlinecalculator.me/finance/savings-goal/) — Find the monthly deposit needed to reach any savings target, given your current balance, interest rate, and time horizon.
- [Simple Interest Calculator](https://onlinecalculator.me/finance/simple-interest/) — Calculate interest earned or owed on a principal amount using the simple interest formula I = Prt.
- [Social Security Calculator](https://onlinecalculator.me/finance/social-security/) — Estimate your monthly Social Security benefit at any retirement age using the 2024 SSA bend point formula, and find the break-even ages between claiming strategies.

### Health

- [Blood Alcohol Content (BAC) Calculator](https://onlinecalculator.me/health/blood-alcohol/) — Estimate blood alcohol content using the Widmark formula. Enter drinks consumed, body weight, and time elapsed to get an estimated BAC percentage.
- [BMI Calculator](https://onlinecalculator.me/health/bmi/) — Calculate your body mass index from height and weight in imperial or metric units, with CDC weight classification and healthy weight range for your height.
- [BMR Calculator](https://onlinecalculator.me/health/bmr/) — Calculate your Basal Metabolic Rate using three formulas — Mifflin-St Jeor, Harris-Benedict (revised), and Katch-McArdle (requires body fat %) — in imperial or metric units.
- [Body Fat Percentage Calculator](https://onlinecalculator.me/health/body-fat/) — Estimate your body fat percentage using the US Navy circumference method. Enter height, neck, waist, and hip measurements to get body fat %, fat mass, lean mass, and a fitness category.
- [Calorie Calculator](https://onlinecalculator.me/health/calorie/) — Estimate daily calorie needs using the Mifflin-St Jeor BMR formula with your activity level and weight goal. Includes macro breakdown for protein, carbs, and fat.
- [Calories Burned Calculator](https://onlinecalculator.me/health/calories-burned/) — Estimate calories burned during exercise using MET values from the Compendium of Physical Activities. Covers 25+ activities.
- [Daily Protein Needs Calculator](https://onlinecalculator.me/health/protein-needs/) — Calculate your daily protein intake target from body weight, activity level, and goal — maintain, muscle gain, or weight loss. Results show total grams, per-meal amount, and g/kg rate.
- [Daily Water Intake Calculator](https://onlinecalculator.me/health/water-intake/) — Calculate your recommended daily water intake from body weight, activity level, and climate. Results in liters, fluid ounces, cups, and milliliters.
- [Gestational Age Calculator](https://onlinecalculator.me/health/gestational-age/) — Calculate gestational age in weeks and days from your last menstrual period (LMP) or from a known ultrasound measurement. Shows due date, trimester, days until delivery, and upcoming milestones.
- [Heart Rate Zones Calculator](https://onlinecalculator.me/health/heart-rate-zones/) — Calculate your 5 heart rate training zones using the Karvonen heart rate reserve method or simple percentage of max HR, based on your age and resting heart rate.
- [Ideal Body Weight Calculator](https://onlinecalculator.me/health/ideal-weight/) — Find your ideal body weight using four evidence-based formulas — Devine, Robinson, Miller, and Hamwi. See the average, the full range, and a side-by-side formula comparison.
- [Lean Body Mass Calculator](https://onlinecalculator.me/health/lean-body-mass/) — Estimate your lean body mass (LBM) using the Boer, James, and Hume formulas. Optionally enter body fat percentage for a direct measurement.
- [Macro Calculator](https://onlinecalculator.me/health/macros/) — Calculate your daily protein, carbohydrate, and fat targets in grams from your calorie goal, body weight, and fitness goal.
- [Max Heart Rate Calculator](https://onlinecalculator.me/health/max-hr/) — Calculate your maximum heart rate using four formulas — Standard (220−age), Tanaka, Gulati, and Fox — and see target heart rate zones for any intensity level.
- [Ovulation Calculator](https://onlinecalculator.me/health/ovulation/) — Calculate your ovulation date, fertile window, and next period from your last menstrual period date, cycle length, and luteal phase. See how many days until ovulation and whether you are currently in your fertile window.
- [Period Calculator](https://onlinecalculator.me/health/period/) — Track and predict your next periods, ovulation dates, and fertile windows for up to 6 future cycles. Enter your last period date, cycle length, and period duration to see your full menstrual cycle calendar.
- [Pregnancy Due Date Calculator](https://onlinecalculator.me/health/pregnancy/) — Estimate your due date using Naegele's rule from the first day of your last period or conception date. Shows gestational week, trimester, days remaining, and progress.
- [Race Time Predictor](https://onlinecalculator.me/health/race-time-predictor/) — Predict finish times for 1 mile, 5K, 10K, half marathon, and marathon from any known race performance using the Riegel formula.
- [Running Pace Calculator](https://onlinecalculator.me/health/running-pace/) — Calculate your running pace per mile or per kilometer, speed in mph and kph, and projected finish times for common race distances.
- [Sleep Calculator](https://onlinecalculator.me/health/sleep/) — Find the ideal bedtime or wake time based on 90-minute sleep cycles. Enter when you need to wake up or when you plan to go to bed to get recommendations for 5, 5.5, or 6 complete sleep cycles.
- [TDEE Calculator](https://onlinecalculator.me/health/tdee/) — Calculate Total Daily Energy Expenditure from your BMR and activity level. Set a weight goal to get your calorie target and macro breakdown.
- [VO2 Max Calculator](https://onlinecalculator.me/health/vo2-max/) — Estimate your VO2 max from a Rockport walk test, Cooper 12-minute run, resting heart rate, or 1.5-mile run. Shows fitness category and approximate percentile.
- [Waist-to-Hip Ratio Calculator](https://onlinecalculator.me/health/waist-hip-ratio/) — Calculate your waist-to-hip ratio (WHR) and find your cardiovascular risk category using World Health Organization thresholds.

### Math

- [Average Calculator – Mean, Median, Mode](https://onlinecalculator.me/math/average/) — Find the mean, median, mode, range, min, max, count, and sum for any list of numbers. Paste or type values separated by commas, spaces, or new lines.
- [Circle Calculator](https://onlinecalculator.me/math/circle/) — Calculate circle radius, diameter, circumference, and area from any one known measurement.
- [Combinations and Permutations Calculator](https://onlinecalculator.me/math/combinations/) — Calculate C(n,k) combinations (order doesn't matter) and P(n,k) permutations (order matters) from n items taken k at a time.
- [Distance Formula Calculator](https://onlinecalculator.me/math/distance-formula/) — Calculate the distance between two points in a coordinate plane. Also returns the midpoint and slope of the connecting line.
- [Exponent Calculator](https://onlinecalculator.me/math/exponent/) — Calculate any base raised to any power, including negative exponents, fractional exponents for roots, and very large numbers shown in scientific notation.
- [Factorial Calculator](https://onlinecalculator.me/math/factorial/) — Calculate n! for any non-negative integer. Shows the exact value for small n and scientific notation for large factorials with digit count.
- [Fraction Calculator](https://onlinecalculator.me/math/fraction/) — Add, subtract, multiply, and divide fractions. Results are automatically simplified to lowest terms with the mixed number and decimal equivalent shown.
- [GCD and LCM Calculator](https://onlinecalculator.me/math/gcd-lcm/) — Find the greatest common divisor (GCD) and least common multiple (LCM) for two or more integers. Supports any number of inputs.
- [GPA Calculator](https://onlinecalculator.me/math/gpa/) — Calculate your term GPA from letter grades and credit hours. Optionally add your existing GPA and credits to compute your updated cumulative GPA.
- [Grade Calculator](https://onlinecalculator.me/math/grade/) — Calculate your overall grade percentage, letter grade, and GPA from assignment scores. Supports weighted averages and plus/minus grading scales.
- [Leap Year Calculator](https://onlinecalculator.me/math/leap-year/) — Check whether any year is a leap year and find the next and previous leap years using the Gregorian calendar rules.
- [Logarithm Calculator](https://onlinecalculator.me/math/logarithm/) — Calculate log base 2, natural log (ln), log base 10, or any custom base logarithm. Uses the change-of-base formula and shows the expression, result, and all three common log values simultaneously.
- [Midpoint Calculator](https://onlinecalculator.me/math/midpoint/) — Find the midpoint, distance, slope, and line equation for any two coordinate points. Enter x and y values for both points and get instant results.
- [Mixed Number Calculator](https://onlinecalculator.me/math/mixed-number/) — Add, subtract, multiply, and divide mixed numbers. Converts each mixed number to an improper fraction, performs the operation, simplifies by GCD, and converts the result back to a mixed number with a decimal equivalent.
- [Nth Root Calculator](https://onlinecalculator.me/math/nth-root/) — Calculate the nth root of any real number, from square roots to 100th roots. Shows the result, whether it is a perfect root, and step-by-step working.
- [Number Base Converter](https://onlinecalculator.me/math/number-base/) — Convert integers between binary, octal, decimal, hexadecimal, or any custom base from 2 to 36. See all four standard representations at once.
- [Percent Error Calculator](https://onlinecalculator.me/math/percent-error/) — Calculate percent error between a measured (experimental) value and the accepted (theoretical) value. Also shows absolute and relative error.
- [Percentage Calculator](https://onlinecalculator.me/math/percentage/) — Five common percentage problems in one place — find a percent of a number, express one number as a percent of another, calculate percent change, or increase/decrease a value by a percentage.
- [Percentage Change Calculator](https://onlinecalculator.me/math/percentage-change/) — Calculate percentage change between two values, find the new value after a percentage increase or decrease, or work backwards to find the original value.
- [Pressure Converter](https://onlinecalculator.me/math/pressure/) — Convert between 12 pressure units including Pascal, psi, bar, atm, mmHg, and more.
- [Prime Factorization Calculator](https://onlinecalculator.me/math/prime-factorization/) — Find the prime factorization of any positive integer. Shows prime factors, exponent notation, number of divisors, and all divisors.
- [Proportion Calculator](https://onlinecalculator.me/math/proportion/) — Solve for any missing value in a proportion A/B = C/D. Enter three known values and find the fourth.
- [Quadratic Equation Solver](https://onlinecalculator.me/math/quadratic/) — Solve any quadratic equation ax² + bx + c = 0. Shows the discriminant, solution type, and both real or complex roots instantly.
- [Range and IQR Calculator](https://onlinecalculator.me/math/range-iqr/) — Calculate the range, quartiles Q1–Q3, interquartile range (IQR), and outliers for any data set. Enter values separated by commas.
- [Ratio Calculator](https://onlinecalculator.me/math/ratio/) — Simplify ratios, find the decimal equivalent, and solve proportions. Supports two-term and three-term ratios.
- [Right Triangle Calculator](https://onlinecalculator.me/math/right-triangle/) — Solve a right triangle from any two known values — sides or angles. Returns all sides, both acute angles, area, and perimeter.
- [Roman Numeral Converter](https://onlinecalculator.me/math/roman-numeral/) — Convert between Arabic numbers and Roman numerals instantly. Supports numbers from 1 to 3999 with a full symbol breakdown.
- [Scientific Calculator](https://onlinecalculator.me/math/scientific/) — A full-featured scientific calculator with trigonometry, logarithms, exponents, factorials, memory functions, and keyboard support. Degrees and radians mode.
- [Slope Calculator](https://onlinecalculator.me/math/slope/) — Calculate slope, y-intercept, line equation, distance, midpoint, and angle from two coordinate points. Supports all line types.
- [Square Root Calculator](https://onlinecalculator.me/math/square-root/) — Calculate the square root, cube root, or any nth root of any real number. Shows whether the result is a perfect root, and includes the value squared and cubed for reference.
- [Standard Deviation Calculator](https://onlinecalculator.me/math/standard-deviation/) — Calculate population and sample standard deviation, variance, and mean from a list of numbers. Enter values separated by commas, spaces, or new lines.
- [Triangle Calculator](https://onlinecalculator.me/math/triangle/) — Solve any triangle from any combination of sides and angles using the law of sines and law of cosines. Returns all sides, angles, area, and perimeter.
- [Variance Calculator](https://onlinecalculator.me/math/variance/) — Calculate population or sample variance, standard deviation, mean, median, mode, and range from a list of numbers. Enter values separated by commas.
- [Volume Calculator](https://onlinecalculator.me/math/volume/) — Calculate volume and surface area for cubes, rectangular boxes, spheres, cylinders, cones, and rectangular pyramids. Works in cm, m, inches, or feet.
- [Z-Score & Normal Distribution Calculator](https://onlinecalculator.me/math/z-score/) — Calculate z-scores from raw values, convert z-scores to percentiles, or find the z-score for any percentile using the standard normal distribution.

### Unit Converters

- [Acceleration Converter](https://onlinecalculator.me/unit-converters/acceleration/) — Convert between 8 acceleration units including m/s², ft/s², standard gravity (g), galileo, and km/h per second. Enter a value, choose units, and get the full conversion table instantly.
- [Angle Converter](https://onlinecalculator.me/unit-converters/angle/) — Convert between degrees, radians, gradians, turns, arcminutes, arcseconds, and NATO mils instantly.
- [Area Converter](https://onlinecalculator.me/unit-converters/area/) — Convert area between square millimeters, square centimeters, square meters, square kilometers, square inches, square feet, square yards, acres, and hectares.
- [Celsius to Fahrenheit Calculator](https://onlinecalculator.me/unit-converters/celsius-fahrenheit/) — Convert temperature instantly between Celsius, Fahrenheit, and Kelvin. Includes a quick-reference table of common temperatures.
- [Clothing Size Converter](https://onlinecalculator.me/unit-converters/clothing-size/) — Convert clothing sizes for women's tops, men's tops, women's pants, men's pants, and women's dresses between US, UK, EU, IT, FR, and AU sizing systems.
- [Cooking Measurement Converter](https://onlinecalculator.me/unit-converters/cooking/) — Convert between 12 cooking volume units including US cups, tablespoons, teaspoons, fluid ounces, milliliters, liters, and UK measures. Enter a value, choose units, and get the full table instantly.
- [Data Storage Converter](https://onlinecalculator.me/unit-converters/data-storage/) — Convert between bits, bytes, kilobytes, megabytes, gigabytes, terabytes, and binary equivalents (KiB, MiB, GiB, TiB).
- [Data Transfer Rate Converter](https://onlinecalculator.me/unit-converters/data-rate/) — Convert between 10 data rate units including Mbps, Gbps, kbps, bytes per second, kibibits, mebibits, and more. Enter a value, choose units, and get the full conversion table instantly.
- [Density Converter](https://onlinecalculator.me/unit-converters/density/) — Convert between 10 density units including kg/m³, g/cm³, lb/ft³, lb/in³, slug/ft³, and more. Enter a value, choose units, and get the full conversion table instantly.
- [Energy Converter](https://onlinecalculator.me/unit-converters/energy/) — Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, and electronvolts. Covers physics, nutrition, and electricity.
- [Force Converter](https://onlinecalculator.me/unit-converters/force/) — Convert between 8 force units including Newton, kilonewton, pound-force, kilogram-force, dyne, and kip. Enter a value, choose units, and get the full conversion table instantly.
- [Frequency Converter](https://onlinecalculator.me/unit-converters/frequency/) — Convert between Hz, kHz, MHz, GHz, THz, RPM, radians per second, degrees per second, and BPM instantly.
- [Fuel Economy Converter](https://onlinecalculator.me/unit-converters/fuel-economy/) — Convert fuel economy between MPG (US), MPG (UK), km/L, L/100km, and L/100mi instantly. See all equivalents in a single table. Useful for comparing cars across countries.
- [Length Converter](https://onlinecalculator.me/unit-converters/length/) — Convert length between millimeters, centimeters, meters, kilometers, inches, feet, yards, and miles. See all unit equivalents at once.
- [Paper Size Converter](https://onlinecalculator.me/unit-converters/paper-size/) — Convert paper sizes between millimeters, centimeters, inches, and pixels (96 dpi) for A4, Letter, Legal, A3, and 13 other standard sizes. Get orientation, aspect ratio, and pixel dimensions instantly.
- [Pounds to Kilograms Calculator](https://onlinecalculator.me/unit-converters/pounds-kilograms/) — Convert weight between pounds, kilograms, ounces, grams, stone, milligrams, metric tons, and short tons using exact NIST conversion factors.
- [Power Converter](https://onlinecalculator.me/unit-converters/power/) — Convert between watts, kilowatts, megawatts, horsepower, PS, BTU per hour, and more power units instantly.
- [Shoe Size Converter](https://onlinecalculator.me/unit-converters/shoe-size/) — Convert shoe sizes between US Men's, US Women's, UK, EU, CM (foot length), and JP systems instantly. Enter your size, select the system, and see all equivalents from a verified lookup table.
- [Speed Converter](https://onlinecalculator.me/unit-converters/speed/) — Convert speed between mph, km/h, m/s, knots, ft/s, and Mach number. Useful for travel, aviation, physics, and everyday comparisons.
- [Temperature Converter](https://onlinecalculator.me/unit-converters/temperature/) — Convert between Celsius, Fahrenheit, Kelvin, and Rankine instantly. See all four units at once.
- [Time Converter](https://onlinecalculator.me/unit-converters/time-convert/) — Convert time between milliseconds, seconds, minutes, hours, days, weeks, months, and years. Useful for programming, project planning, and everyday calculations.
- [Torque Converter](https://onlinecalculator.me/unit-converters/torque/) — Convert between 8 torque units including Newton-meter, pound-force foot, kilogram-force meter, and more. Enter a value, choose units, and get the full conversion table instantly.
- [Weight and Mass Converter](https://onlinecalculator.me/unit-converters/weight-mass/) — Convert between grams, kilograms, pounds, ounces, stone, metric tons, and US and long tons instantly.

### Date & Time

- [Add or Subtract Days Calculator](https://onlinecalculator.me/date-time/add-subtract-days/) — Add or subtract any number of days from a starting date and instantly see the resulting date and day of the week.
- [Age Calculator](https://onlinecalculator.me/date-time/age/) — Calculate exact age in years, months, and days from a date of birth, with total days, total weeks, and days until your next birthday.
- [Business Days Calculator](https://onlinecalculator.me/date-time/business-days/) — Count the number of business days (Monday–Friday) between two dates, alongside calendar days and weekend days.
- [Countdown Calculator](https://onlinecalculator.me/date-time/countdown/) — Live countdown to any future date and time, updated every second, showing days, hours, minutes, and seconds remaining.
- [Date Difference Calculator](https://onlinecalculator.me/date-time/date-difference/) — Count the exact number of days, weeks, months, business days, hours, and minutes between any two dates.
- [Daylight Duration Calculator](https://onlinecalculator.me/date-time/daylight-duration/) — Calculate daylight hours for any latitude across a full year or a custom date range. See monthly averages, the longest and shortest days, and solstice and equinox dates.
- [Days Until Holiday Calculator](https://onlinecalculator.me/date-time/days-until-holiday/) — See how many days until every major US and international holiday, all on one page. Updates automatically based on today's date or any reference date you choose.
- [Hours Calculator](https://onlinecalculator.me/date-time/hours-calculator/) — Add up timesheet hours and minutes across multiple shifts, subtract breaks, and get the total in hours and decimal format.
- [Julian Date Converter](https://onlinecalculator.me/date-time/julian-date/) — Convert between Gregorian calendar dates and Julian Day Numbers (JDN). Also computes the Julian Date, Modified Julian Date, and J2000 epoch offset used in astronomy.
- [Moon Phase Calculator](https://onlinecalculator.me/date-time/moon-phase/) — Find the moon phase for any date — new moon, full moon, or anything in between. Shows phase name, illumination percentage, and the dates of the next new and full moons.
- [Sunrise and Sunset Calculator](https://onlinecalculator.me/date-time/sunrise-sunset/) — Find the exact sunrise, solar noon, and sunset times for any date and location. Also shows civil and astronomical twilight, day length, and polar day or night conditions.
- [Time Adder and Subtractor](https://onlinecalculator.me/date-time/time-adder/) — Add or subtract days, hours, minutes, and seconds from any date and time. Get the exact result date and time instantly. Uses UTC internally so the result is always correct regardless of daylight saving time.
- [Time Duration Calculator](https://onlinecalculator.me/date-time/time-duration/) — Calculate the exact elapsed time between two dates and times in years, months, days, hours, minutes, and seconds.
- [Time Zone Converter](https://onlinecalculator.me/date-time/time-zone-converter/) — Convert a time from one world time zone to another. Supports 18 common zones including US, Europe, Asia, and Pacific.
- [Unix Timestamp Calculator](https://onlinecalculator.me/date-time/unix-timestamp/) — Convert a Unix timestamp to a human-readable date and time, or convert any date and time to its Unix timestamp in seconds and milliseconds.
- [Week Number Calculator](https://onlinecalculator.me/date-time/week-number/) — Find the ISO and US week number for any date, plus day of year, day of week, quarter, and weeks remaining in the year.
- [Weekday Calculator](https://onlinecalculator.me/date-time/weekday/) — Find the day of the week, day of the year, ISO week number, and whether a date is a weekend or falls in a leap year.

### Construction

- [Asphalt Calculator](https://onlinecalculator.me/construction/asphalt/) — Calculate tonnage, volume, and truckloads of hot mix asphalt or bags of cold patch for any driveway or paving project.
- [Brick Calculator](https://onlinecalculator.me/construction/brick/) — Estimate the number of bricks and mortar bags needed for any wall — with door and window openings, standard brick sizes, and waste factor.
- [Carpet Calculator](https://onlinecalculator.me/construction/carpet/) — Calculate carpet square yardage and total cost including padding and installation for any room size.
- [Concrete Block Calculator](https://onlinecalculator.me/construction/concrete-block/) — Calculate how many concrete blocks (CMU) you need for a wall, including courses, blocks per course, waste factor, and mortar bags.
- [Concrete Calculator](https://onlinecalculator.me/construction/concrete/) — Calculate how much concrete you need for slabs, footings, and cylindrical columns in cubic yards, cubic feet, and cubic meters. Also shows how many 60-lb and 80-lb bags to buy.
- [Deck Calculator](https://onlinecalculator.me/construction/deck/) — Estimate decking boards, joists, rim joists, and screws needed for any wood or composite deck — with waste factor.
- [Drywall Calculator](https://onlinecalculator.me/construction/drywall/) — Calculate drywall sheets, joint compound, tape, and screws for any room — including walls and ceiling with waste factor.
- [Excavation Calculator](https://onlinecalculator.me/construction/excavation/) — Calculate bank volume, loose volume, and dump trucks needed for any excavation project — with soil-type swell factors for clay, loam, sand, and rock.
- [Fence Calculator](https://onlinecalculator.me/construction/fence/) — Calculate posts, rails, pickets, concrete bags, and chain-link rolls for any fence — with gate count, post spacing, and waste factor.
- [Flooring Calculator](https://onlinecalculator.me/construction/flooring/) — Calculate how much flooring to buy for any room. Enter room dimensions and a waste percentage to get total square footage, square yards, and an optional box count for packaged flooring products.
- [Framing Calculator](https://onlinecalculator.me/construction/framing/) — Calculate studs, plates, headers, and total 2×4 lumber needed for any wall — with doors, windows, and waste factor.
- [Gravel Calculator](https://onlinecalculator.me/construction/gravel/) — Calculate how much gravel, pea stone, or crushed rock you need. Enter length, width, and depth to get cubic yards, cubic feet, and estimated tons.
- [Gutter Calculator](https://onlinecalculator.me/construction/gutter/) — Calculate gutter length, downspouts, elbows, end caps, and joint connectors for any roof — with size recommendation and waste factor.
- [Insulation Calculator](https://onlinecalculator.me/construction/insulation/) — Calculate bags or rolls of insulation needed for any area and R-value — covers blown-in fiberglass, cellulose, batts, and spray foam.
- [Lumber Calculator](https://onlinecalculator.me/construction/lumber/) — Calculate board feet from lumber dimensions. Add multiple sizes and quantities to get total board feet, piece count, and lineal feet.
- [Mulch Calculator](https://onlinecalculator.me/construction/mulch/) — Calculate how much mulch you need in cubic yards, cubic feet, and bags for any garden bed or landscape area.
- [Paint Calculator](https://onlinecalculator.me/construction/paint/) — Calculate how many gallons of paint you need for any room. Enter wall area, number of coats, coverage rate, and waste percentage to get an exact gallon count and a rounded-up purchase quantity.
- [Rebar Calculator](https://onlinecalculator.me/construction/rebar/) — Calculate the number of rebar bars, total linear footage, weight, and estimated cost for concrete slabs — with bar size and spacing options.
- [Roofing Calculator](https://onlinecalculator.me/construction/roofing/) — Calculate shingle bundles, roofing squares, ridge cap, and nails for any roof — with pitch factor, waste, and shingle type.
- [Sand Calculator](https://onlinecalculator.me/construction/sand/) — Calculate how much sand you need in cubic yards, cubic feet, and tons for any leveling, playground, patio, or construction project.
- [Siding Calculator](https://onlinecalculator.me/construction/siding/) — Calculate siding squares, boards, or panels for any house — with door and window deductions, exposure setting, and waste factor.
- [Square Footage Calculator](https://onlinecalculator.me/construction/square-footage/) — Calculate the square footage of one or more rooms in rectangle, circle, or triangle shapes. Supports feet, inches, meters, and yards with totals in all four unit systems.
- [Stair Calculator](https://onlinecalculator.me/construction/stair/) — Calculate step count, actual riser height, stringer length, and tread boards for any staircase — with IRC code compliance check.
- [Tile Calculator](https://onlinecalculator.me/construction/tile/) — Calculate how many tiles you need for any room. Enter room dimensions, tile size in inches, grout joint width, and waste percentage to get a tile count with an extra-tile buffer.
- [Topsoil Calculator](https://onlinecalculator.me/construction/topsoil/) — Calculate how much topsoil you need in cubic yards, cubic feet, and tons for any garden, lawn, or raised-bed project.
- [Wallpaper Calculator](https://onlinecalculator.me/construction/wallpaper/) — Calculate how many rolls of wallpaper you need for a room, accounting for wall area, doors, windows, and pattern repeat waste.

### Everyday

- [Allowance Calculator](https://onlinecalculator.me/everyday/allowance/) — Calculate a recommended weekly, monthly, and annual allowance based on your child's age, chore hours, or a percentage of household income.
- [Bill Split Calculator](https://onlinecalculator.me/everyday/bill-split/) — Split a restaurant bill evenly across any number of people, including tip. See how much each person owes.
- [Carbon Footprint Calculator](https://onlinecalculator.me/everyday/carbon-footprint/) — Estimate your personal annual carbon footprint from driving, electricity, natural gas, and flights using EPA emission factors.
- [Cat Age Calculator](https://onlinecalculator.me/everyday/cat-age/) — Convert your cat's age to the human equivalent — or find the cat age for any human age. Includes life stage from Kitten to Geriatric.
- [Chore Schedule Value Calculator](https://onlinecalculator.me/everyday/chore-schedule/) — Calculate the monetary value of household chores to set allowances or understand the economic worth of unpaid household labor.
- [Coffee Savings Calculator](https://onlinecalculator.me/everyday/coffee-savings/) — Calculate how much you save per year by making coffee at home instead of buying at a café, including compounded investment growth over 5, 10, and 20 years.
- [Cooking Time Calculator](https://onlinecalculator.me/everyday/cooking-time/) — Calculate oven cooking times for chicken, turkey, beef, pork, lamb, salmon, and baked goods based on weight, cooking method, and doneness.
- [Dice Roller & Probability Calculator](https://onlinecalculator.me/everyday/dice/) — Roll any combination of dice (d4, d6, d8, d10, d12, d20, d100) with a modifier, and calculate the probability of rolling at or above a target number.
- [Dog Age Calculator](https://onlinecalculator.me/everyday/dog-age/) — Convert your dog's age to the human equivalent. Results adjust for dog size because small dogs age more slowly than large breeds.
- [Electricity Cost Calculator](https://onlinecalculator.me/everyday/electricity-cost/) — Calculate how much an appliance or device costs to run. Enter wattage, hours per day, days, and your electricity rate.
- [Fuel Cost Calculator](https://onlinecalculator.me/everyday/fuel-cost/) — Calculate the total fuel cost for any trip. Enter distance, fuel efficiency, and price per gallon or liter to see total cost, fuel used, and cost per mile or kilometer in both MPG and L/100km modes.
- [Mileage Calculator](https://onlinecalculator.me/everyday/mileage/) — Calculate your car's fuel economy in MPG plus cost per mile and cost per 100 miles from a fill-up.
- [Ohm's Law Calculator](https://onlinecalculator.me/everyday/ohm/) — Calculate voltage, current, resistance, and power from any two known values using Ohm's Law (V = I × R) and the power equations (P = V × I).
- [Oven Temperature Converter](https://onlinecalculator.me/everyday/oven-temp/) — Convert oven temperatures between Fahrenheit, Celsius, and Gas Mark instantly. Includes a full reference table and plain-English descriptions like Cool, Moderate, Hot.
- [Phone Plan Comparison Calculator](https://onlinecalculator.me/everyday/phone-plan/) — Compare the total cost of keeping your current phone plan versus switching, including upfront fees, early termination charges, and the break-even timeline.
- [Random Number Generator](https://onlinecalculator.me/everyday/random-number/) — Generate one or more random integers or decimal numbers within any range, with options to allow or prevent duplicate values.
- [Reading Time Calculator](https://onlinecalculator.me/everyday/reading-time/) — Find out how long it takes to read any text. Enter a word count and reading speed to get minutes and seconds.
- [Recipe Scale Calculator](https://onlinecalculator.me/everyday/recipe-scale/) — Scale any recipe up or down by changing the number of servings. Enter original and desired servings and adjust your ingredient list.
- [Road Trip Cost Calculator](https://onlinecalculator.me/everyday/road-trip-cost/) — Calculate the total fuel cost of a road trip plus cost per person and cost per mile from your car's MPG and the gas price.
- [Tip Calculator](https://onlinecalculator.me/everyday/tip/) — Calculate tip amount, total bill, and per-person split for any party size. Quick preset buttons for common tip percentages.
- [Typing Speed Calculator](https://onlinecalculator.me/everyday/typing-speed/) — Calculate your WPM, CPM, accuracy, and adjusted WPM from a typing test. Enter words typed, characters, time in minutes, and number of errors.
- [Water Usage and Cost Calculator](https://onlinecalculator.me/everyday/water-cost/) — Estimate your household's daily water usage and monthly water bill based on showers, baths, toilet flushes, dishwasher loads, laundry, and outdoor watering.

## Articles

Plain-English guides on how people run the numbers — common mistakes, shortcuts, and what each calculator assumes. One article per day; each links to its calculator.

- [Stop Believing These 8 Standard Deviation Myths](https://onlinecalculator.me/blog/standard-deviation-myths/) — Standard deviation is a powerful tool for understanding data, but it is easy to misinterpret. Learn the truth behind eight common misunderstandings so you can choose the right formula and accurately read your results.
- [5 Percentage Change Rules to Do Math in Your Head](https://onlinecalculator.me/blog/percentage-change-rules-of-thumb/) — Learn five reliable mental math tricks to quickly estimate percentage increases and decreases. These practical rules of thumb will help you calculate raises, tips, and discounts without a calculator.
- [5 Fraction Examples to Master Basic Math](https://onlinecalculator.me/blog/fraction-examples/) — Learn how to add, subtract, multiply, and divide fractions without relying on decimals. These five real-world examples show you exactly how to apply fraction arithmetic to cooking, carpentry, and everyday problem-solving.
- [7 Hidden Factors That Alter Room Square Footage](https://onlinecalculator.me/blog/what-affects-square-footage/) — Measuring a room is rarely as simple as length times width. Discover the seven factors that warp your final square footage, from wall thickness and sloped ceilings to material waste and rounding errors.
- [5 Steps to Find Your TDEE and Calorie Target](https://onlinecalculator.me/blog/how-to-use-a-tdee-calculator/) — Learn how to calculate your Total Daily Energy Expenditure to figure out exactly how many calories you burn each day. This step-by-step guide explains how to use your BMR, activity level, and weight goals to find your ideal calorie and macro targets.
- [9 Mistakes to Avoid When Calculating Your Due Date](https://onlinecalculator.me/blog/pregnancy-calculator-mistakes/) — A simple data entry error or misunderstanding of gestational math can throw off your estimated due date by weeks. Learn the most common errors people make when calculating their pregnancy timeline and how to get the right numbers.
- [How to Calculate Your Macros in 3 Steps](https://onlinecalculator.me/blog/how-macros-is-calculated/) — Learn the exact math behind setting up a proper macro split. By anchoring your protein to your body weight, setting a healthy baseline for fat, and filling the rest with carbs, you can optimize your diet for fat loss or muscle growth.
- [6 Facts About Ideal Body Weight Formulas](https://onlinecalculator.me/blog/ideal-weight-checklist/) — Ideal body weight formulas were originally designed for hospital drug dosing, not fitness goals. Learn how these calculations work, why they ignore muscle mass, and how to use them as a helpful reference rather than a strict rule.
- [4 Reasons Your Calorie Calculator Is Wrong](https://onlinecalculator.me/blog/calories-burned-calculator-limits/) — Fitness trackers and apps use standard MET formulas to estimate energy expenditure, but individual factors like body composition, mechanical efficiency, and the environment can throw those numbers off by up to 20 percent. Here is how to actually use the data.
- [How to calculate your daily calorie needs](https://onlinecalculator.me/blog/what-is-a-typical-calorie/) — The standard 2,000-calorie diet isn't right for everyone. Learn how to calculate your Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE) to find the exact number of calories your body actually needs to maintain, lose, or gain weight.
- [5 Common Questions About Body Fat Percentage](https://onlinecalculator.me/blog/body-fat-questions-answered/) — Learn the difference between total weight and body fat, why men and women have different healthy ranges, and how to accurately measure your body composition at home using just a flexible tape measure.
- [5 Steps to Calculate Your BMR by Hand](https://onlinecalculator.me/blog/how-to-calculate-bmr-by-hand/) — Learn how to calculate your basal metabolic rate manually. We break down the math behind the Mifflin-St Jeor and Katch-McArdle formulas so you can understand exactly how your age, height, weight, and body composition drive your daily energy needs.
- [Stop Believing These 5 Roth IRA Myths](https://onlinecalculator.me/blog/roth-ira-myths/) — Misunderstandings about contribution limits, taxes, and income rules often cause investors to leave money on the table. Clearing up these five common Roth IRA misconceptions makes it easier to build a retirement strategy that actually works.
- [7 Essential Retirement Rules of Thumb](https://onlinecalculator.me/blog/retirement-rules-of-thumb/) — You do not need a massive spreadsheet to figure out if you are on track for retirement. These seven mental shortcuts offer a fast way to estimate your savings targets, asset mix, and withdrawal limits without getting stuck in the math.
- [3 Real-World Mortgage Refinance Scenarios](https://onlinecalculator.me/blog/refinance-examples/) — Refinancing your mortgage can lower your monthly payment or save you thousands in interest, but closing costs complicate the math. Explore three detailed scenarios to see how different loan terms and rate drops affect your bottom line.
- [The 5 Factors That Decide How Much House You Can Afford](https://onlinecalculator.me/blog/what-affects-home-affordability/) — Lenders use a strict math equation to determine your maximum purchase price. Learn how your income, debts, interest rates, down payment, and local taxes impact your borrowing power.
- [How to Calculate Store Discounts and Compare Deals](https://onlinecalculator.me/blog/how-to-use-a-discount-calculator/) — Stop doing mental math in the shopping aisle. Learn how to quickly figure out exact savings, compare percentage versus dollar-off deals, and calculate stacked discounts before you reach the register.
- [5 Mistakes That Ruin Your Auto Loan Math](https://onlinecalculator.me/blog/auto-loan-calculator-mistakes/) — A car payment calculator is only as accurate as the numbers you enter. Learn how to avoid common errors with sales tax, dealer fees, and interest rates so you get a realistic estimate before heading to the dealership.
- [The Simple Math Behind Your 401(k) Growth](https://onlinecalculator.me/blog/how-401k-is-calculated/) — Projecting your retirement savings might feel like a guessing game, but it relies on a straightforward formula. Learn how contributions, employer matches, market returns, and inflation work together to build your future wealth.
- [How to Calculate Exact Age: 5 Factors](https://onlinecalculator.me/blog/age-checklist/) — Figuring out your exact age in years, months, and days requires more than simple subtraction. Learn how to account for leap years, target dates, and calendar quirks to get precise results for legal and medical forms.
- [3 Hidden Assumptions in Your Tip Calculator](https://onlinecalculator.me/blog/tip-calculator-limits/) — Tipping tools do the math perfectly, but they rely on default settings that might not match your meal. Learn how to handle uneven splits, automatic gratuities, and pre-tax versus post-tax tipping to get the right number every time.
- [Master Your Scientific Calculator Functions](https://onlinecalculator.me/blog/what-is-a-typical-scientific/) — Learn how to use advanced math functions like trigonometry, logarithms, and factorials. Discover what normal answers look like so you can catch mistakes and calculate with confidence.
- [7 formulas to solve common percentage problems](https://onlinecalculator.me/blog/percentage-questions-answered/) — Learn the exact math formulas to calculate tips, discounts, salary raises, and percent changes. This guide breaks down the seven most common percentage problems with easy real-world examples.
- [A Complete Guide to Calculating Your GPA by Hand](https://onlinecalculator.me/blog/how-to-calculate-gpa-by-hand/) — Learn the math behind your academic performance. This guide explains how to convert letter grades to points, weigh them by credit hours, and figure out your term and cumulative GPA without relying on an automated system.
- [5 BMI Myths Debunked: What the Math Really Means](https://onlinecalculator.me/blog/bmi-myths/) — Body mass index is a simple math equation, but it is often misunderstood. Learn the truth behind five common BMI myths and discover what this number actually tells you about your health, body fat, and weight categories.
- [6 Mental Math Tricks for Estimating Sales Tax](https://onlinecalculator.me/blog/sales-tax-rules-of-thumb/) — Learn how to quickly estimate sales tax in your head before you reach the checkout counter. These practical mental math tricks help you calculate final costs, handle discounts, and reverse-engineer receipts with ease.
- [How Gross Pay Becomes Net Pay: 3 Examples](https://onlinecalculator.me/blog/paycheck-examples/) — See exactly how taxes and deductions shrink your salary into take-home pay. We break down the math for single earners, married couples, and high earners to help you understand your net income.
- [5 Factors That Determine Your Mortgage Payment](https://onlinecalculator.me/blog/what-affects-mortgage/) — A mortgage payment is the result of a simple math equation combined with a few real-world variables. Learn how your principal, interest rate, loan term, extra payments, and escrow costs work together to determine your monthly housing budget.
- [Master Your Debt With a Loan Calculator](https://onlinecalculator.me/blog/how-to-use-a-loan-calculator/) — Learn the exact steps to calculate your monthly loan payments, understand your amortization schedule, and see how making extra payments can save you thousands in interest.
- [Demystifying the Compound Interest Formula](https://onlinecalculator.me/blog/how-compound-interest-is-calculated/) — Learn the mechanics of compound interest. We break down the standard formula, explain how monthly contributions accelerate your savings, and show why compounding frequency matters for your financial goals.
- [11 Mistakes Ruining Your Tax Calculations](https://onlinecalculator.me/blog/income-tax-calculator-mistakes/) — A tax calculator is only as accurate as the numbers you provide. Learn how to avoid eleven common errors—like confusing gross and net pay or forgetting local taxes—so you can get a true picture of your take-home pay and budget with confidence.
- [Avoid These Compound Interest Calculator Mistakes](https://onlinecalculator.me/blog/compound-interest-calculator-mistakes/) — Compound interest calculators are powerful tools for projecting your financial future, but only if you use them correctly. Learn about the most common errors people make when running their numbers and how to keep your estimates grounded in reality.
- [10 Mortgage Calculator Mistakes](https://onlinecalculator.me/blog/mortgage-calculator-mistakes/) — Most people type in a price and a rate, then trust the monthly payment that pops out. Here are the ten things that quietly make that number wrong — and the fix for each.

## URL structure

All calculator pages follow the pattern: `https://onlinecalculator.me/{category}/{calculator-id}/`

Categories: `finance`, `health`, `math`, `unit-converters`, `date-time`, `construction`, `everyday`

Share-with-values URLs append inputs as query params, e.g.:
`/finance/mortgage/?principal=400000&rate=7.25&term=360&extra=0`
All inputs can be pre-filled this way.

## What the site does NOT cover

- AMT (Alternative Minimum Tax), tax credits (child tax credit, earned income credit, etc.)
- Self-employment business accounting beyond SE tax
- Real-time currency conversion (exchange rates change; we use fixed formulas only)
- Net Investment Income Tax (NIIT) — the 3.8 % surtax on investment income above the AGI threshold
- State income tax with its own progressive brackets — the calculator uses a flat-rate approximation

## Site metadata

- Domain: onlinecalculator.me
- Language: English (US conventions, US units default)
- Deployed on Cloudflare Pages
- Built with Astro 4 (static output) + Tailwind CSS
- Generated: 2026-07-19

## Machine-readable content

- Every calculator page has a Markdown mirror: drop the trailing slash and append `.md` — e.g. `https://onlinecalculator.me/finance/mortgage.md`
- Blog articles too: `https://onlinecalculator.me/blog/<slug>.md`
- [llms-full.txt](https://onlinecalculator.me/llms-full.txt) — the full text of every calculator page in one Markdown file
- [RSS feed](https://onlinecalculator.me/blog/rss.xml) — one practical article per day
- **MCP server**: `https://onlinecalculator.me/mcp` (Streamable HTTP, no auth) — tools `list_calculators`, `describe_calculator`, `calculate` run all 175 calculators server-side with the same tested math

## Other discoverability pages

Browse pages that index calculators by other axes:

- [Homepage](https://onlinecalculator.me/) — search, popular calculators, favorites, browse by category
- [All calculators (A–Z)](https://onlinecalculator.me/calculators/) — every published calculator in one static page, grouped by category
- [Finance](https://onlinecalculator.me/finance/) — 29 calculators in finance
- [Health](https://onlinecalculator.me/health/) — 23 calculators in health
- [Math](https://onlinecalculator.me/math/) — 35 calculators in math
- [Unit Converters](https://onlinecalculator.me/unit-converters/) — 23 calculators in unit converters
- [Date & Time](https://onlinecalculator.me/date-time/) — 17 calculators in date & time
- [Construction](https://onlinecalculator.me/construction/) — 26 calculators in construction
- [Everyday](https://onlinecalculator.me/everyday/) — 22 calculators in everyday

## Static pages

- [About](https://onlinecalculator.me/about/) — what the site is and how it works
- [Privacy](https://onlinecalculator.me/privacy/) — no tracking, no personal data stored