# Free online proportion calculator

> A proportion states that two ratios are equal. Enter three values in A/B = C/D and this calculator finds the fourth using cross-multiplication.

Solve for any missing value in a proportion A/B = C/D. Enter three known values and find the fourth.

**Interactive version:** https://onlinecalculator.me/math/proportion/
**Category:** Math

## How to use

1. Enter three of the four values: A, B, C, and D.
2. Select which variable to **solve for**.
3. Click **Solve Proportion**.
4. See the result and a verification check.

## Formula

**A/B = C/D** — cross multiply: A × D = B × C

| Solve for | Formula |
|-----------|---------|
| A | A = B × C / D |
| B | B = A × D / C |
| C | C = A × D / B |
| D | D = B × C / A |

## Worked example

**Recipe scaling:** A recipe calls for 3 cups flour for 4 servings. How much for 10 servings?

- A = 3 (cups), B = 4 (servings), D = 10, solve for C
- C = A × D / B = 3 × 10 / 4 = **7.5 cups**

**Unit rate:** If a car travels 60 miles in 2 hours, how far in 5 hours?

- A = 60, B = 2, D = 5, solve for C
- C = 60 × 5 / 2 = **150 miles**

## Frequently asked questions

### How do you solve a proportion?

Cross-multiply and divide. For A/B = C/D, rearrange to find the missing variable: A = B×C/D, B = A×D/C, C = A×D/B, D = B×C/A.

### What is cross-multiplication?

Cross-multiplication means A×D = B×C. This is true for any valid proportion. To find D, divide both sides by A.

### What is a proportion used for?

Proportions appear everywhere — scaling recipes, converting units, calculating percentages, map scales, similar triangles, and speed/distance/time problems.

### What is the difference between ratio and proportion?

A ratio compares two quantities (3:4). A proportion is a statement that two ratios are equal (3/4 = 6/8). This calculator solves proportion equations.

### Can a proportion have zero values?

The denominator (B or D) cannot be zero. Numerators and other variables can theoretically be zero, but most practical applications use positive values.

## Sources

- [Proportionality (mathematics)](https://en.wikipedia.org/wiki/Proportionality_(mathematics)) — Wikipedia
- [Directly Proportional](https://mathworld.wolfram.com/DirectlyProportional.html) — Wolfram MathWorld

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- [Percentage calculator](https://onlinecalculator.me/math/percentage/) — Calculate percentages and percentage change.
- [GCD and LCM calculator](https://onlinecalculator.me/math/gcd-lcm/) — Find the greatest common divisor and least common multiple.

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