CalcPenguin

Ratio Calculator

Solve a proportion A : B = C : D for the missing value, and simplify a ratio to whole numbers, with the working shown.

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Solves A : B = C : D for D.

D

20

A : B in simplest form
3 : 4
A ÷ B
0.75

3 : 4 is the same ratio as 15 : 20.

How it's worked out

  1. Scale factor from A to C15 ÷ 35
  2. D = B × scale factor4 × 520
  3. Simplify A : Bdivide both by 13 : 4

The rule

A : B = C : D → D = B × C ÷ A

Worked example

A recipe uses 3 cups of flour to 4 cups of milk. With 15 cups of flour you need 4 × 15 ÷ 3 = 20 cups of milk.

Questions people ask

How do I solve a ratio?

Find how much A was scaled to make C, then scale B by the same amount. 3 : 4 = 15 : D, so D = 4 × 15 ÷ 3 = 20.

How do I simplify a ratio?

Divide both sides by their greatest common divisor. 12 : 18 simplifies to 2 : 3.

Where are ratios used?

Recipes, mixing concrete or paint, map scales, and sharing money in proportion.

Formula and rules last checked 25 October 2026Written and tested by the CalcPenguin teamReport a problem