a, b, c
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How this calculator works
Two ratios are in proportion when they are equal. Given three of the four numbers, the fourth follows by cross-multiplying. This is the rule of three used for scaling recipes, converting between units, reading maps and working out prices for different quantities.
Formula
- a / b = c / x
- Cross-multiply: a × x = b × c
- x = b × c / a
Worked example
3 : 4 = 9 : x
- a
- 3
- b
- 4
- c
- 9
Resultx = 12
3 : 4 = 9 : 12.
Frequently asked questions
How do I solve a proportion?
Cross-multiply and divide. For 3 / 4 = 9 / x, multiply 4 by 9 and divide by 3 to get x = 12.
What is the rule of three?
Another name for the same method: if a corresponds to b, then c corresponds to b × c / a.
Does this work for inverse proportion?
No. For inverse proportion, where one quantity falls as the other rises, the products are equal instead: a × b = c × x.
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Results are estimates based on the values you enter and the published method shown above. They do not replace professional medical, financial or legal advice. Method reviewed October 7, 2026. About this site