Rule of Three Calculator
Enter three values and get the fourth instantly — for direct proportions (more means more) and inverse proportions (more means less), with the formula shown.
A corresponds to B. How much does C correspond to? (More of A means more of X.)
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How to use
- Pick the mode: proportional (3 kg cost €12 — what do 5 kg cost?) or inverse (3 workers need 12 days — how long do 5 workers need?).
- Enter the three known values A, B and C.
- Read the result X — the formula used is displayed below it.
What is this tool?
The rule of three is the classic technique for solving proportions: given three known values in a proportional relationship, it computes the fourth. The direct version scales up and down (price per quantity, recipe amounts, currency at a fixed rate); the inverse version handles relationships where more of one means less of the other (workers vs. time, speed vs. travel time).
Common uses
- Scaling recipe quantities to a different number of servings
- Computing prices: if 3 items cost X, what do 7 cost?
- Homework: classic direct and inverse proportion problems
- Estimating job durations when the team size changes
FAQ
When do I use the inverse rule of three?
When more of one quantity means less of the other: more workers → less time, higher speed → shorter travel time, more machines → fewer days. If more means more, use the proportional mode.
Can I use decimals?
Yes — both comma and dot are accepted as decimal separators, and results show up to four decimal places.
What's the formula behind it?
Proportional: X = B × C ÷ A. Inverse: X = A × B ÷ C. The calculator shows the applied formula with your numbers below the result.