Proportion Calculator
Solve a/b = c/x for the missing value using cross-multiplication.
How to use the Proportion Calculator
- Enter a.
- Enter b.
- Enter c.
- Your results update instantly: x (so that a/b = c/x), ratio a : b, scale factor.
What is a proportion?
A proportion says two ratios are equal, for example 3/4 = 9/12. If you know three of the four values you can always find the fourth.
Cross-multiplication
a / b = c / x → a × x = b × c → x = (b × c) ÷ a
Example: a recipe uses 3 cups of flour for 4 people. For 9 people? Write 4/3 = 9/x… or simply scale: 3 × 9 ÷ 4 = 6.75 cups.
Everyday uses
- Scaling recipes and mixing ratios.
- Map scales and model sizes.
- Currency and unit conversions.
- Similar triangles in geometry.
Frequently asked questions
How do you solve a proportion?
Cross-multiply the two ratios and divide by the number next to the unknown.
What is a direct proportion?
Two quantities are in direct proportion when one doubles as the other doubles, so their ratio stays constant.
Last updated 15 November 2026. Results are estimates for general information and are not financial, tax, legal or medical advice.
