Tool

Proportion Calculator

The "rule of three": if you know three values in a proportion, find the fourth. Great for scaling recipes, mixing ratios, prices, and more.

If…

gives

(result of A)

then…

gives

Answer

—

Direct vs. inverse

Direct: more of one means more of the other (2 boards cost $10 → 5 boards cost $25). x = B × C ÷ A.
Inverse: more of one means less of the other (2 workers take 10 hours → 5 workers take 4 hours). x = A × B ÷ C.