Proportion Calculator (Rule of Three)
Averages and the rule of three
Formula: x = b × c ÷ a · Updated Oct 22, 2026
If A corresponds to B, what does C correspond to? Enter three values and pick direct (more → more) or inverse (more → less) proportion.
How to use the Proportion Calculator
- Enter A and the B that goes with it.
- Enter the new value C.
- Choose direct or inverse and read X.
Direct proportion
X = B × C ÷ A. If 3 kg cost $4.50, 7 kg cost 4.50 × 7 ÷ 3 = $10.50.
Inverse proportion
X = A × B ÷ C. If 4 workers need 6 days, 3 workers need 4 × 6 ÷ 3 = 8 days.
Frequently asked questions
What is the rule of three?
A way to find the fourth value of a proportion from three known ones by cross-multiplying.
When is a proportion inverse?
When doubling one value halves the other: workers and days, speed and travel time.
More calculators
Results are rounded for display; the calculation itself uses full precision. Tax and price results are estimates — check the official rate for your case.