Proportion Calculator
Solve proportions and ratios — find the missing value in A/B = C/D.
Enter any three values and leave one field empty — the missing value will be solved automatically using cross-multiplication.
A
/B
=C
/D
Solved for D:12
3 / 4 = 9 / 12
3 : 4 = 9 : 12
Solving proportions with cross-multiplication
A proportion states that two ratios are equal: A/B = C/D. To find a missing value, cross-multiply: A × D = B × C. Rearranging for the unknown gives: A = B×C/D, B = A×D/C, C = A×D/B, or D = B×C/A. Proportions are used in scaling recipes, maps, unit conversions, similar triangles, and currency exchange. Leave any single field blank and this calculator finds the missing value instantly — all math runs locally in your browser.
Private & free — this tool runs entirely in your browser.
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