Proportion Solver (a/b = c/x)

About this calculator

This tool solves for the unknown value x in a proportion of the form a/b = c/x by cross-multiplying (a × x = b × c) and rearranging for x, the standard technique for any problem stating that two ratios are equal. Proportions like this describe situations where two quantities scale together at a fixed rate, so knowing three of the four values always pins down the fourth.

It is used by students working through ratio and proportion problems in math class, and in everyday situations like scaling a recipe up or down, converting a map scale into a real-world distance, adjusting a mixture's ingredient ratio for a different batch size, or figuring out how much of one currency or unit corresponds to another given a known exchange rate. Anywhere a "if this much corresponds to that much, then how much corresponds to this new amount" question comes up, this is the underlying calculation.