What is a proportion calculator?
A proportion calculator solves equations of the form a/b = c/d when one of the four values is missing. Enter the three numbers you know, leave the unknown box blank (or write a letter such as x), and the calculator finds the missing value and shows every cross multiplication step.
How to use this calculator
- Enter three numbers in any of the fraction boxes.
- Leave the missing value box blank or type a letter (for example x).
- Press Calculate and wait a moment for the result.
- Read the answer and the step-by-step cross multiplication breakdown, then copy or download as a PDF.
Proportion formula
a / b = c / d → a × d = b × c
This rule is called cross multiplication. Multiply the numerator of each fraction by the denominator of the other, set the two products equal, and divide to isolate the unknown.
Solved example
Find x in 4/5 = 6/x. Cross multiplying gives 4 × x = 5 × 6, so x = 30 / 4 = 7.5.