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
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Minimum 0
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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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- Basic Calculator
- Scientific 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.
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