Proportion Calculator
Solve for the missing value in a proportion (A is to B as C is to X).
About the Proportion Calculator
A proportion states that two ratios are equal — "A is to B as C is to X" — and solving for the missing fourth term is a skill used constantly in recipe scaling, map reading, unit conversion, and classroom math. This calculator solves for X directly.
How It Works
The calculator applies cross-multiplication: since A/B equals C/X in a valid proportion, cross-multiplying gives A×X = B×C, and solving for X gives X = (B×C)/A.
If A/B = C/X, then X = (B × C) / A.
Example
Scenario: Solving 4 is to 9 as 12 is to X.
Result: X = (9 × 12) / 4 = 27.
Assumptions & Limitations
- Assumes A is nonzero, since dividing by zero is undefined.
- Assumes the proportion is set up as A/B = C/X, matching the standard 'A is to B as C is to X' phrasing.
- If your problem is phrased differently (like A/B = X/C), rearrange the terms before entering them to match this calculator's A, B, C, X structure.
Frequently Asked Questions
What is cross-multiplication?
Cross-multiplication is the technique of multiplying the numerator of one ratio by the denominator of the other and setting the products equal, which is the standard way to solve any proportion for a missing term.
Where are proportions used in everyday life?
Proportions come up in recipe scaling, currency conversion, map scale reading, mixing solutions to the right concentration, and countless other situations where two quantities must stay in a fixed relationship.
Can proportions have negative numbers?
Yes — the same cross-multiplication math works with negative values, as long as none of the terms in the denominators are zero.