What is the rule of three?
The rule of three finds a missing value when three others are known. In a direct proportion, more of one thing means proportionally more of the other, for example price and quantity: if 3 items cost 12, then 5 cost 12 × 5 ÷ 3 = 20.
In an inverse proportion, more of one means less of the other, such as workers and time: if 4 workers need 6 days, 8 workers need 4 × 6 ÷ 8 = 3 days.
How to use the Rule of Three Calculator
- 1Choose direct or inverse.
- 2Enter A, B and C.
- 3Read the missing value.
Frequently asked questions
How do I know which type to use?
If both values grow together it is direct. If one grows while the other shrinks, it is inverse.
Can I use decimals?
Yes. Comma and dot both work.