How to calculate your BMR by hand in 5 steps

Your basal metabolic rate, or BMR, is the absolute minimum number of calories your body needs to stay alive over a 24-hour period. Think of it as the energy required for vital baseline functions like breathing, circulating blood, and repairing cells, assuming you spend the entire day at complete, uninterrupted rest.

Online tools can spit this number out in seconds. But walking through the math by hand shows you exactly how age, height, weight, and body composition drive your metabolism. Doing the calculations yourself builds a much clearer picture of your actual energy needs.

Step 1: Convert your measurements to metric

All three major BMR formulas rely on the metric system. If you track your weight in pounds and your height in inches, you have to convert those numbers to kilograms and centimeters first. Plugging imperial numbers directly into these equations will give you wildly inaccurate results.

You also need your current age in whole years. If you want to calculate your BMR based on your lean muscle mass, you will need a relatively accurate estimate of your body fat percentage.

Step 2: Choose the right formula

Three primary equations exist for calculating a basal metabolic rate. Each serves a slightly different purpose depending on your body composition and the data you have available.

The Mifflin-St Jeor equation from 1990 is the recommended formula for most people. It is widely considered the gold standard for general adults and serves as the preferred equation of the Academy of Nutrition and Dietetics. For most healthy people, it predicts measured BMR within about ±10%.

The original Harris-Benedict equation goes all the way back to 1919, though it was revised by Roza and Shizgal in 1984 to improve accuracy. You will still see it used frequently in clinical settings, though it tends to estimate a slightly higher metabolic rate than Mifflin-St Jeor.

The Katch-McArdle formula takes a completely different approach. It ignores your total weight, height, and age entirely. Instead, it calculates BMR strictly from your lean body mass. This makes it highly accurate for athletes carrying above-average muscle mass or people with very low body fat, where standard weight-based formulas tend to fall short.

Step 3: Calculate BMR using Mifflin-St Jeor

Because it is the most validated formula for general use, start with Mifflin-St Jeor. The equations differ slightly for men and women.

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5 Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

Let’s run the numbers for a 30-year-old male who weighs 80 kg and is 180 cm tall.

1. Multiply weight by 10: 10 × 80 = 800
2. Multiply height by 6.25: 6.25 × 180 = 1125
3. Multiply age by 5: 5 × 30 = 150
4. Combine those results: 800 + 1125 − 150 = 1775
5. Add the male constant: 1775 + 5 = 1780 kcal per day.

If we calculate this for a woman with the exact same physical measurements, the first four steps remain identical. In the final step, we simply subtract 161 instead of adding 5, giving us 1614 kcal per day.

Step 4: Calculate BMR using Katch-McArdle

If you know your body fat percentage, you can use the Katch-McArdle formula. First, you calculate your Lean Body Mass (LBM).

LBM = Weight in kg × (1 − (body fat % ÷ 100)) BMR = 370 + (21.6 × LBM)

Take the same 80 kg male, and assume he has a body fat percentage of 15%.

1. Convert body fat to a decimal: 15 ÷ 100 = 0.15
2. Subtract from 1: 1 − 0.15 = 0.85
3. Multiply by total weight to find LBM: 80 × 0.85 = 68 kg
4. Multiply LBM by the formula constant: 21.6 × 68 = 1468.8
5. Add the baseline constant: 370 + 1468.8 = 1838.8 kcal per day. (You can round this to 1839 kcal.)

Notice that this result comes out slightly higher than the Mifflin-St Jeor estimate. Because this specific person has a relatively lean body composition, his extra muscle mass demands a bit more baseline energy to maintain.

Step 5: Turn your BMR into TDEE

Your BMR is only part of the daily calorie equation. Since BMR assumes zero movement, digestion, or activity, you burn more calories than your BMR every single day just by getting out of bed and walking to the kitchen.

To find your actual maintenance calories, you have to factor in your lifestyle. This is called your Total Daily Energy Expenditure (TDEE). You calculate it by multiplying your BMR by an activity factor. These multipliers usually range from 1.2 for a completely sedentary lifestyle up to 1.9 for extreme daily exercise or a highly physical job. Once you have your baseline metabolic rate, running it through a TDEE calculator or a comprehensive Calorie calculator will give you the complete picture of your daily energy needs.

When to let the calculator do the math

Working out these formulas on paper is a great way to grasp the relationship between your physical stats and your metabolism. You can clearly see that adding height or weight increases your energy needs, while aging gradually decreases them.

But calculating by hand quickly introduces friction. The revised Harris-Benedict formula, for example, relies on highly specific decimals:

Men: 88.362 + (13.397 × Weight) + (4.799 × Height) − (5.677 × Age) Women: 447.593 + (9.247 × Weight) + (3.098 × Height) − (4.330 × Age)

If you ran our 80 kg, 180 cm, 30-year-old male through this equation, you would get an estimate of roughly 1934 kcal. Tracking all those decimal points on a scratchpad leaves plenty of room for basic arithmetic errors.

Your BMR is also not a static number. If you actively lose weight or put on muscle, your baseline energy requirements will shift. Recalculating these complex equations manually every time your scale moves gets tedious. A dedicated tool handles the strict order of operations instantly, toggles between imperial and metric units without manual conversion, and lets you compare all three formulas side-by-side in milliseconds.

Ready to skip the scratchpad and find your baseline instantly? Use the BMR Calculator.