Tools / Rule of three calculator
Rule of three calculator
If A gives B, what does C give? Choose proportional (more → more) or inverse proportional (more → less, e.g. workers and time).
corresponds to B
then Ccorresponds to
Calculation
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How to use it
- Enter the known pair: A corresponds to B.
- Enter the new value C.
- Choose proportional or inverse – the result and the steps are shown immediately.
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Last checked: October 2026. Found a mistake? Use the form below – we fix confirmed errors and update the date.
Frequently asked questions
When is it proportional?
When doubling one value doubles the other – e.g. 3 kg cost €6, so 5 kg cost €10.
When is it inverse?
When doubling one value halves the other – e.g. 4 workers need 6 days, so 8 workers need 3 days.
What is the formula?
Proportional: X = B × C ÷ A. Inverse: X = A × B ÷ C.