Ratio Calculator
Find the missing number in a ratio or proportion.
Solving a proportion
Two ratios are equal when they describe the same relationship. If A : B = C : X, then cross-multiplying gives X = B × C ÷ A.
Example
If 3 : 4 = 9 : X, then X = 4 × 9 ÷ 3 = 12. The numbers are in the same relationship, because 3 goes to 9 by multiplying by 3, and so does 4 to 12.
Real uses
- Scaling a recipe: if 3 cups serve 4 people, how many cups serve 9?
- Map and model scales.
- Mixing paint, fuel or fertiliser to a stated ratio.
Frequently asked questions
Can I use decimals?
Yes. Any numbers work, as long as A is not zero.
What is the difference between a ratio and a fraction?
A ratio compares two quantities, and a fraction is part of a whole. They are written alike, so A : B and A ÷ B carry the same information.
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