Proportion Solver

About this calculator

A proportion states that two ratios are equal, written as a/b = c/x, and is solved for the unknown term using cross-multiplication: a × x = b × c, so x equals (b × c) divided by a. This cross-multiplication technique is one of the first algebraic manipulation methods taught in school arithmetic, built on the principle that multiplying both sides of an equation by the same value preserves equality.

Proportions underpin countless everyday and professional calculations — scaling a recipe up or down, converting between units, resizing an image while keeping its aspect ratio, calculating map or blueprint scale, determining dosage per body weight in medicine, and solving similar-triangle problems in geometry all reduce to solving a proportion for a missing term.

This calculator takes any three known terms of a/b = c/x and instantly solves for the missing fourth value.