Proportion Calculator

A proportion is an equation that says two ratios are equal: A : B = C : D, equivalently A ÷ B = C ÷ D. When three of the four values are known, the fourth is fixed and can be solved with cross multiplication.

Multiply both sides of A ÷ B = C ÷ D by B and by D to clear the denominators. The result is A × D = B × C. Whichever of A, B, C, or D is missing, isolate it on one side to get the missing value. The calculator runs this rearrangement automatically.

Leave the field blank or type x (or ?). Exactly one field should be left blank. If you leave more than one blank, the proportion has too few knowns to solve; if you fill in all four, the calculator just checks whether the proportion is true.

The proportion is undefined when the denominator on the side opposite to the missing value is 0. The calculator detects this and flags it. To avoid the issue, double-check the three known values for typos.

A ratio is one comparison: A : B. A proportion is an equation between two ratios: A : B = C : D. The ratio calculator simplifies a single ratio; the proportion calculator solves for an unknown value in an equality between two ratios.

Anywhere you scale a known relationship. Recipe scaling (if 2 cups feed 4 people, how many cups feed 6?), map scales, unit conversions, percent problems (the proportion 'is over of equals percent over 100'), and similar-triangle geometry. Any time two quantities change together at a constant rate, a proportion captures that rate.