Math
Proportion Calculator
Solve A/B = C/D for the missing value.
Inputs
Proportion Calculator
Result
D
12
Cross-multiplication in one click
A proportion says two ratios are equal: a/b = c/d. Given any three values, the fourth is fixed by cross-multiplication. This underpins scaling recipes, unit conversion, map distances, and similar-triangle geometry.
Related math
Reduce and compare ratios with the Ratio Calculator; simplify the resulting fractions with the Fraction Calculator.