Proportion Calculator
Solve A : B = C : X for the missing value X.
Formula: A / B = C / X → X = B × C / A
Worth Knowing
Solving a proportion relies on cross-multiplication: if A/B equals C/X, then A times X equals B times C, which rearranges to find the unknown. This single idea powers a surprising amount of everyday math — converting units, scaling recipes, reading map distances, and computing similar-triangle lengths all reduce to solving a proportion. The method works only when the two ratios genuinely vary together in direct proportion; inversely related quantities need a different setup.
How to Use
- Enter A into the first field of the Proportion Calculator.
- Fill in the rest: B, C.
- Your result appears instantly as you type — there's no submit button or page reload.
Formula
A / B = C / X → X = B × C / A