Rule of three calculator

In a direct rule of three, if one quantity goes up the other goes up; in an inverse one, if one goes up the other goes down (e.g. workers and time).

This rule of three calculator solves a proportion instantly: tell it that A equals B, give it C, and it works out X. Switch between direct and inverse proportions with one click.

Proportions made simple

Perfect for recipes, scaling quantities, unit pricing, percentages and quick estimates. The calculation runs entirely in your browser.

FAQ

What is the rule of three?

It's a way to find an unknown value from a proportion: if A relates to B, then C relates to X. You solve for X by cross-multiplying.

Direct or inverse — which do I use?

Use direct when both quantities move the same way (more hours, more pay). Use inverse when they move oppositely (more workers, less time).

How is X calculated?

Direct: X = B × C ÷ A. Inverse: X = A × B ÷ C. The tool applies the right formula based on your choice.