Proportion Calculator – Solve a/b = c/d

Understanding proportions and when to use this calculator

A proportion states that two ratios are equal: a/b = c/d. When three of the four values are known, you can find the fourth using cross-multiplication. This is useful in cooking (scaling a recipe from 4 servings to 6), in geometry (similar triangles), in unit conversion (miles to kilometres), and in many real-world problems where one quantity scales with another.

The proportion calculator accepts decimal and whole numbers. Enter a, b, and c to solve for d. Ensure b (the denominator of the first ratio) is not zero, and a is not zero when solving for d. For simplifying ratios rather than solving proportions, use our ratio calculator. For percentage problems, use the percentage calculator.

Proportions appear everywhere: in maps (scale 1:10000 means 1 cm on the map equals 10000 cm on the ground), in finance (interest proportional to principal), and in science (concentration proportional to amount of solute). Whenever two quantities maintain a constant ratio, a proportion can describe the relationship. This calculator handles the classic case where you know three parts of the equation and need the fourth.

This tool runs in your browser and does not store your inputs. It is free to use and requires no sign-up. For related tools, try our ratio calculator to simplify ratios, or our percentage calculator for percent increase, decrease, and other percentage problems.