Proportion Calculator
Solve a proportion a / b = c / x for the unknown value x.
The unknown that solves a / b = c / x.
- 1
Cross multiply
5 × 8 = 40From a/b = c/x, cross multiplying both sides gives a × x = b × c. - 2
Divide by a
40 ÷ 2 = 20
Formula
How this is calculated
A proportion is the statement that two ratios are equal: a / b = c / x. Here a, b, and c are the three known numbers you enter, and x is the unknown the calculator solves for. The two ratios are assumed equivalent, meaning the left fraction reduces to the same value as the right fraction.
The solution comes from cross multiplication. Multiplying both sides of a / b = c / x by b and by x clears the denominators and gives a · x = b · c. Dividing both sides by a isolates the unknown, so x = (b × c) ÷ a. The calculator plugs your three values straight into this formula and shows the result to four decimal places.
The numbers are pure (unitless) ratios, so any consistent unit cancels out. The only requirement is that a is not zero — dividing by zero leaves x undefined, which the tool flags. It also assumes the unknown sits in the bottom-right slot; if yours is elsewhere, flip or swap the ratios first, since equal ratios stay equal when rearranged.
Examples
| Input | Result |
|---|---|
| a = 2, b = 5, c = 8 (2/5 = 8/x) | x = 20 |
About this calculator
A proportion states that two ratios are equal, such as a / b = c / x. When three of the four values are known, the fourth can be found by cross multiplication: multiply diagonally and solve. For the form a / b = c / x this gives a · x = b · c, so x = (b × c) ÷ a.
Proportions appear everywhere from cooking and map scales to converting units and scaling recipes or models. As long as a is not zero, the proportion has exactly one solution. This calculator assumes the unknown is in the bottom-right position; rearrange your ratio to match if your unknown is elsewhere.
Frequently asked questions
Cross multiplication multiplies each numerator by the opposite denominator. For a / b = c / x it gives a·x = b·c, which you solve for x.
Cross multiply to get a·x = b·c, then divide both sides by a. The result is x = (b × c) ÷ a, valid whenever a is not zero.
Rearrange the ratio so the unknown sits in the bottom-right (the x slot). Equal ratios can be flipped or swapped without changing the proportion.
Also known as
TG we-Calculate Editorial Team. (2026). Proportion Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/proportion-calculator
TG we-Calculate Editorial Team. "Proportion Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/proportion-calculator.
TG we-Calculate Editorial Team, "Proportion Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/proportion-calculator
@misc{wecalculate_proportion_calculator, title = {Proportion Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/proportion-calculator}}, year = {2026}, note = {TG we-Calculate} }
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