Rule of Three Calculator
Enter three known values to find the fourth. Use direct proportion when both quantities grow together, and inverse proportion when one grows as the other shrinks.
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If A gives B, then C gives X. Example: 3 kg costs $12, so how much do 5 kg cost?
How to Use
- 1Choose direct or inverse proportion.
- 2Enter A and B, a pair of values you already know (for example, 3 kg costs 12).
- 3Enter C, the new value of the first quantity (for example, 5 kg).
- 4X, the missing value, appears instantly, with the formula used to get it.
Frequently Asked Questions
What is the rule of three?
It's a quick way to find a missing value when four quantities are proportional. If you know three of them, the fourth follows from cross-multiplication: X = (B × C) ÷ A for a direct proportion.
How do I know if a proportion is direct or inverse?
Ask whether the quantities move in the same direction. More items cost more money, so that's direct. More workers finish a job faster, so fewer days are needed, and that's inverse.
Can I use decimals?
Yes. You can type decimals with either a point or a comma, such as 2.5 or 2,5.