Rule of three calculator
Solve a simple rule of three (direct or inverse): enter three values and find the fourth.
X (result)
25
Proportion2 / 10 = 5 / X
How the calculation works
The rule of three finds an unknown value from a proportion.
Direct (both grow together): A/B = C/X → X = (B × C) / A
Inverse (one grows, the other shrinks): A × B = C × X → X = (A × B) / C
Examples
- If 2 kg cost $10, how much for 5 kg? Direct → X = (10×5)/2 = 25.
Frequently asked questions
When to use the inverse rule of three?
When increasing one quantity decreases the other — e.g., more workers, less time to finish.
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Updated on June 17, 2026 · by Rafael Rossi · Methodology & sources