Cross Multiplication Calculator

About this calculator

Cross multiplication is the standard technique for solving a proportion of the form a/b = c/x: you multiply diagonally across the equals sign — a times x and b times c — to get a×x = b×c, then divide to isolate the unknown. It works because two fractions are equal exactly when their cross products match, a fact that follows directly from multiplying both sides by the denominators.

The technique shows up everywhere ratios do: scaling a recipe up or down, converting between units, reading a map scale to find real-world distance, calculating medication dosage from a known concentration, or solving for a missing side in similar triangles. Because it turns a proportion into a single linear equation, it is usually faster than clearing fractions by hand.

This calculator takes your known values of a, b and c, cross multiplies them, and solves directly for the missing x.