Proportion Calculator

Solve for a missing value in a proportion a/b = c/d

a (a/b = c/d)

About this calculator

A proportion states that two ratios are equal: a/b = c/d. To find the missing value, use cross-multiplication: d = (b × c) / a. Proportions are fundamental in cooking (scaling recipes), maps (converting distances), and any situation involving equivalent rates or ratios.

How to use

If 3 workers build 4 meters of wall per day, how far do 9 workers build? a=3, b=4, c=9 → d = (4×9)/3 = 12 meters. The proportion 3/4 = 9/12 holds.

Frequently asked questions

What is cross-multiplication?

In a proportion a/b = c/d, cross-multiplying gives a × d = b × c. This allows you to solve for any one unknown given the other three values.