Math
Ratio Calculator
Solve a proportion A : B = C : D for the missing value, and simplify a ratio to whole numbers, with the working shown.
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
- Scale factor from A to C
15 ÷ 35 - D = B × scale factor
4 × 520 - Simplify A : B
divide 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.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem