Proportion Calculator
Solve a proportion a / b = c / x for the unknown value x.
The unknown that solves a / b = c / x.
Formula
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.
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