Ratio Calculator

It divides both sides of the ratio by their greatest common divisor (GCD), found with the Euclidean algorithm. For example, 48:36 reduces to 4:3 because the GCD is 12.

Given three of the four values in the proportion a/b = c/d, the calculator finds the fourth via cross-multiplication. Pick which value is unknown from the dropdown.

A zero in a denominator position makes the proportion undefined. The calculator shows a dash (—) and flags the proportion as invalid so you can fix the input.