Rule of Three Calculator
Solve direct and inverse proportion problems fast with this rule of three calculator, useful for scaling ratios, recipes and unit conversions in your browser.
Enter Values
Minimum 0
Minimum 0
Minimum 0
How to use this calculator
- Enter the three known values A, B and C.
- Press Calculate to find X, where A is to B as C is to X.
How it works
For a direct proportion A : B = C : X, the unknown is X = B × C ÷ A. It's the classic cross-multiply method for scaling quantities.
Worked example
3 : 12 = 5 : X. X = 12 × 5 ÷ 3 = 20.
Common mistakes
- Crossing the wrong pair — keep the known pair in line and solve for the matching unknown.
- Using it for non-proportional relationships, where doubling one doesn't double the other.
Frequently asked questions
What is the rule of three?
A way to find an unknown in a proportion: if a relates to b, then c relates to x as x = (b × c) ÷ a.
When can I use the rule of three?
Only when the quantities are directly (or inversely) proportional — like price per item, or speed and time over a fixed distance.
What's the rule of three used for?
Scaling recipes, converting quantities, working out unit prices, map distances — anything where two amounts stay in proportion.
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- Percentage Calculator
- Basic Calculator
- Scientific Calculator
- Proportion Calculator
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Explore more in Maths, Education & Science.
Cite this tool
Zerdly. (2026). Rule of Three Calculator. Retrieved from https://www.zerdly.com/tools/rule-of-three-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.