Solve the unknown number in the proportion by cross-multiplying: a : b = c : x.
The Proportion Calculator solves the unknown number in a proportion. A proportion means the equality of two ratios, and the unknown is solved by cross-multiplying.
The calculator is free, works in your browser without registration, and shows the result as soon as you change the values. The calculation is done on your own device, and the information you enter is not sent anywhere.
If a : b = c : x, then a × x = b × c and x = b × c / a.
3 : 12 = 5 : x → x = 12 × 5 / 3 = 20. With default values, the calculator gives the result: x 20.0000.
For students and everyday calculations, such as scaling a recipe, map scales, and currency conversions.
A proportion is an equation where two ratios are equal, for example, a : b = c : x.
Multiply the opposite terms together: a × x = b × c. Then solve for x.
For scales, changing quantities in recipes, and price calculations.