✓ Effortless Math · Free Tool

Ratio & Proportion Calculator

Solve a proportion a/b = c/d when exactly one value is unknown — leave a box blank (or type ?) and get the missing value by cross-multiplication, with the simplified ratio, unit rate, and full step-by-step working.

Proportion:  a ∶ b = c ∶ d  (i.e. a ÷ b = c ÷ d)
a b = c d

Leave exactly one box blank (or type ?) and we'll solve for it. Decimals and negatives are fine; no value may be 0 in a way that divides.

Results

Missing value (x)
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Simplified ratio (a ∶ b)
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Unit rate (a ÷ b)
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Completed proportion
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Step-by-step (cross-multiplication)