# Proportion Calculator

URL: https://speedycalculator.com/calculators/math/proportion/
Last updated: 2026-10-25

> The proportion calculator solves a/b = c/x for the missing value using cross multiplication. It is useful for unit conversions, scaling, map distances and dosage-style problems.

## Inputs

- a
- b
- c

## Results

- x

## Formula

```
x = b × c ÷ a
```

## Explanation

Two ratios are proportional when they are equal. Knowing three of the four values lets you find the fourth.

### How to use the Proportion Calculator

- **a**
- **b**
- **c**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Multiply diagonally across the equals sign and divide by the remaining value.

### Inputs explained

FieldWhat to enteraA numberbA numbercA number

### Example values

How the main result changes when **a** changes and everything else stays at the example values above:

ax1.5642.2542.66673323.7525.64.521.3333616

## Worked example

Inputs — a: 3; b: 8; c: 12.

Results — **x = 32**.

## FAQ

**What is cross multiplication?**
In a/b = c/x, a × x = b × c.

**When do proportions not apply?**
When quantities do not change at a constant rate.

**What formula does the Proportion Calculator use?**
x = b × c ÷ a.