Proportion Calculator
Solve a proportion A/B = C/D for any missing value by cross-multiplying.
About the Proportion Calculator
A proportion says two ratios are equal. Cross-multiplying turns it into A × D = B × C, so any one value can be found from the other three. It's the maths behind scaling recipes, unit prices, maps and currency.
The formula
A/B = C/D ⇔ A × D = B × C
Worked example
3/4 = 9/D
- 3 × D = 4 × 9 = 36
- D = 12
Questions people ask
What's the “rule of three”?
Another name for solving a proportion: if 3 cost 4, then 9 cost 4 × 9 ÷ 3 = 12.
What is an inverse proportion?
When one value goes up as the other goes down, like workers and days: A × B = C × D instead.