45 · Calculators

Ratio Calculator

Fill in any three of the four boxes to solve a proportion — handy for scaling recipes, maps and mixtures.

Solves A : B = C : D. Fill in any three boxes and leave one empty.

Missing value

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How to use the ratio calculator

  1. Enter three values in A : B = C : D.
  2. Leave the one you want to find empty.
  3. Read the missing value and the simplified ratio.

Cross-multiplying

In a proportion, A × D = B × C. If D is missing, D = B × C ÷ A. That’s the “rule of three”: if 3 cups of flour make 4 loaves, 9 cups make 12.

Examples

3 : 4 = 9 : ?

D = 12

2 : 5 = ? : 20

C = 8

Questions

What if I fill in all four?

It checks whether the two ratios are equal and simplifies both.

Can I use decimals?

Yes. Ratios are simplified only when both sides are whole numbers.