Proportion Calculator
Solve for the missing value in a proportion a/b = c/d — the cross-multiplication every scaling problem needs.
Solved proportion
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How it works
If a/b = c/d, then the missing term = (product of the diagonal) ÷ the known termFrequently Asked Questions
How do I solve a proportion?
Cross-multiply: in a/b = c/d, a×d = b×c. Leave one value blank (as 0) and the calculator finds it — if 3/4 = x/20, then x = 3×20÷4 = 15.
What is cross-multiplication?
Multiplying diagonally across the equals sign. It works because two equal fractions have equal cross-products, turning a fraction equation into a simple multiplication you can solve.
Where are proportions used?
Everywhere things scale: recipe conversions, map distances, unit pricing, mixing ratios, and similar-triangle geometry. Any 'if this, then how much of that' question is a proportion.