Proportion Calculator
Solve a proportion a/b = c/x for any missing term by cross-multiplication.
How it works
a/b = c/x → cross-multiply to a × x = b × c → x = b × c ÷ a
A proportion sets two ratios equal — a/b = c/x — and is solved by cross-multiplication. Multiplying diagonally turns the equation into a × x = b × c, and dividing by the coefficient of the unknown isolates it. Pick which of the four terms is missing and the calculator rearranges the same relationship to find it.
Worked example
Solve 2/3 = 4/x. Cross-multiply to get 2 × x = 3 × 4 = 12, so x = 12 ÷ 2 = 6, completing the proportion 2/3 = 4/6. Both ratios reduce to the same value.
Frequently asked questions
What is a proportion?
A statement that two ratios are equal, like a/b = c/x. When three of the four values are known, cross-multiplication finds the fourth.
How does cross-multiplication work?
For a/b = c/x, multiply diagonally to get a × x = b × c, then divide to isolate the unknown. It works because multiplying both sides of an equation by the same amount keeps it balanced.
Where are proportions used?
Scaling recipes, converting units, reading maps, resizing images and figuring unit prices all set one ratio equal to another and solve for the missing piece.
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This calculator is for educational purposes. Double-check important results before acting on them.