Rule of three calculator
Solve direct and inverse rules of three: if A goes with B, what goes with C. Shows the formula with your values so you can see where the result comes from.
Direct: when A grows, B grows in the same proportion.
How it works
How it works
The rule of three solves proportions like “if A goes with B, what goes with C?”. Type the three values and the tool works out X and shows the formula with your numbers.
- Direct: both quantities grow together. If 2 kg cost 10 €, 5 kg cost
X = B × C / A = 10 × 5 / 2 = 25 €. - Inverse: when one grows, the other shrinks. If 4 people take 6 days, 8 people take
X = A × B / C = 4 × 6 / 8 = 3 days.
Which one to pick
Ask yourself what happens to the second quantity if you double the first. If it doubles too (twice the kilos, twice the price), it is direct. If it halves (twice the people, half the days), it is inverse. In the direct rule A cannot be 0, and in the inverse rule C cannot be 0, because you would have to divide by 0.
FAQ
When is it direct and when inverse?
It is direct when one quantity grows as the other grows (more kilos, higher price). It is inverse when one grows as the other shrinks (more workers, fewer days).