Rule of three
Solve direct and inverse proportions.
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Tool guide
Rule of three: how it works
Practical information to understand the result and use this tool correctly.
About Rule of three
The rule of three finds a missing value in a proportion. Direct form: if a relates to b as c relates to x, then x = (b × c) ÷ a.
Inverse rule of three applies when more of one quantity means less of the other (e.g. more workers, fewer days).
Use clear units on all three inputs so the unknown stays meaningful.
How to use it
- 1Choose direct or inverse proportion.
- 2Enter the three known values A, B and C.
- 3Read X and the formula used.
Frequently asked questions
- What is the rule of three formula?
- Direct: x = (b × c) ÷ a. Inverse: x = (a × b) ÷ c. a, b, c are the known values.
- When do I use inverse rule of three?
- When quantities move in opposite directions proportionally — classic case: workforce size vs days to finish a job.
- Can I use decimals?
- Yes. Decimal inputs are supported.