Rule of three calculator
Solve a rule of three (cross-multiplication) to find the fourth value of a proportion, direct or inverse. Useful for adjusting a recipe, calculating a percentage, or a conversion. Calculation done entirely locally, in your browser.
If A corresponds to B, then C corresponds to X.
How it works
The rule of three finds an unknown fourth value from three known values, assuming a proportional relationship. For a direct proportion (A/B = C/X), it's solved by cross-multiplication: X = (B × C) / A. For an inverse proportion (A × B = C × X), X is isolated as X = (A × B) / C.
When should I use a direct proportion and when an inverse proportion?
A direct proportion applies when both quantities increase or decrease together (for example, the total price increases with the quantity purchased). An inverse proportion applies when one quantity increases while the other decreases (for example, the more workers there are on a job site, the fewer days it takes to finish, for the same amount of work).
A concrete example of a direct rule of three?
If 3 kilos of apples cost €12, how much do 7 kilos cost? Set A = 3, B = 12, C = 7, and calculate X = (12 × 7) / 3 = €28. This is exactly the default calculation offered in the example above.
This simple mathematical calculation runs entirely in your browser, with no data sent.