Proportion Calculator
Solve for the missing value in a proportion, such as scaling ingredients or converting rates, using cross-multiplication.
Your results
Calculation breakdown
- X
- X = (B × C) ÷ A
Worked example
Solving 2 is to 5 as 8 is to X: X = (5 × 8) ÷ 2 = 20.
Assumptions
- Solves proportions of the form A is to B as C is to X.
- A cannot be zero as it is the divisor in the cross-multiplication.
How this calculator works
This calculator solves a proportion of the form A is to B as C is to X, using cross-multiplication to find the missing fourth value. It's a quick way to scale recipes, maps or mixing ratios.
Frequently asked questions
What is cross-multiplication?
It's the method of multiplying the numerator of one ratio by the denominator of the other to solve for an unknown value in a proportion.
What if B is zero?
Dividing by zero is not possible, so the calculator shows an error if B is zero.
Can I use this to scale a recipe?
Yes, enter the original quantity and serving size as A and B, and your desired serving size as C to find the new quantity X.
Editorial review and responsibility
Reviewed by the CalcAussie editorial team on 21 September 2026. Review scope: Method, Australian terminology, material assumptions, source links and user-facing limitations.
Calculator results remain estimates based on the figures entered. Check material decisions against current official information and report a suspected error so it can be reproduced and corrected.