Rule of Three Calculator
Solve direct and inverse proportions and see the calculation step by step.
If A corresponds to B, enter a new value C to find X while keeping the same proportion.
Result
Enter A, B, and C to find X.
What is the rule of three?
The rule of three finds an unknown value from three related values. It works with quantities, prices, recipes, distances, time, and many other measurements.
Direct rule of three
A proportion is direct when both quantities rise or fall together. If 5 products cost 1,000, then 8 products cost 1,600.
X = (B × C) ÷ A
Inverse rule of three
A proportion is inverse when one quantity increases while the other decreases. If 4 people take 6 hours for an equivalent task, 8 people would take 3 hours under an equal-productivity assumption.
X = (A × B) ÷ C
Choosing the right proportion
Choose direct if doubling one quantity should double the other. Choose inverse if doubling one should halve the other. Always confirm that proportionality is a reasonable assumption for the real situation.
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