Proportion Calculator: Fast & Simple

Cross-multiplication, solved for you

A proportion a/b = c/d means the two ratios are equal. Whichever of the four letters you leave blank, the calculator isolates it using cross-multiplication — for example, if d is unknown, a×d = b×c, so d = (b×c)/a. All four possible missing-value cases use the same cross-multiplication rule, just rearranged for whichever letter is empty.

Worked example

Say 4 apples cost $6, and you want to know the cost of 10 apples at the same rate. Set up a/b = c/d as 4/6 = 10/d, leaving d blank. The calculator solves d = (b × c) / a = (6 × 10) / 4 = 60 / 4 = 15. Ten apples cost $15 — consistent with the per-apple unit rate of $6 ÷ 4 = $1.50, since 10 × $1.50 = $15.

The four-value check mode

If you fill in all four fields instead of leaving one blank, the tool switches modes: rather than solving for anything, it computes a/b and c/d separately and checks whether they're equal (within a very small floating-point tolerance, since two ratios that are mathematically equal can differ by a tiny rounding error after division). This is useful for verifying a ratio you've already worked out by hand, or checking whether a scaled recipe, drawing, or mixture actually preserves the original proportion.

Typical uses

Proportions come up in recipe scaling (doubling or halving ingredient ratios), scale drawings and maps (inches on paper to real-world distance), unit price comparisons, and similar-triangle geometry problems where corresponding side ratios must match.