Rule of Three & Proportion Calculator
Solve direct and inverse proportions with the classic rule of three.
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How to use it
- Decide whether the quantities grow together (direct) or in opposite directions (inverse).
- Fill in the three known values: two corresponding pairs plus the new value.
- The result shows the formula used, so you can verify it by hand.
- Results update live — press Reset to clear both calculators.
About this tool
Enter three known values to solve the fourth in both direct and inverse proportion, with the formula shown for each case. Useful for recipes, unit pricing, scale conversions and schoolwork — including a worked example to confirm the setup.
Your input never leaves the browser: this tool works entirely offline once the page has loaded.
Frequently asked questions
Direct or inverse — how do I choose?
If both quantities grow together, it is direct (more kg → more price). If one grows while the other shrinks, it is inverse (more workers, fewer days).
Why can a known value not be zero?
The direct formula divides by X₁ and the inverse by X₂; zero would make the proportion undefined, so the tool refuses with a clear message instead of returning infinity.