Rule of Three Calculator
Simple or compound, direct or inverse — solve any proportion and see every step of the logic explained on an interactive chalkboard.
The Math Behind It
Direct: X = (B × C) / A | Inverse: X = (A × B) / C | Compound: X = B × (C/A) × ratio_f2The rule of three is a mathematical operation that solves proportions between quantities. In simple direct proportions, when one quantity increases the other increases proportionally. In simple inverse, when one increases the other decreases. In compound, two independent factors influence the final result simultaneously.
Example
Simple direct: If 4 workers build 20 m² per day, how much do 7 workers build? X = (7 × 20) / 4 = 35 m². Compound: If 4 workers in 3 days build 20 m², how much do 7 workers build in 6 days? X = 20 × (7/4) × (6/3) = 70 m².
Official Sources & Validity
Calculations verified against current legislation.
Common Core State Standards — Mathematics
Valid for: US Education Standard
International System of Units (SI)
Valid for: Global Standard
Frequently Asked Questions
When quantities vary in opposite directions. Example: more workers → less time to finish a job. The larger one value, the smaller the other.