Proportion Calculator
Solve a proportion a/b = c/d for the missing value — enter any three of the four numbers and the calculator cross-multiplies to find the fourth, with the steps.
How to solve a proportion
A proportion states that two ratios are equal: a/b = c/d. When three of the four values are known, the fourth is fixed, and you find it by cross-multiplying — multiply each numerator by the opposite denominator and set the products equal. Leave one box empty above and the calculator solves for it and shows the cross-multiplication.
a/b = c/d ⟶ a × d = b × c
For example, to solve 3/4 = x/20, cross-multiply to 4x = 3 × 20 = 60, so x = 15. Proportions scale recipes, convert units, resize images and read maps — anywhere two quantities keep the same ratio.
Where proportions are used
- Scaling recipes — doubling or halving ingredient amounts.
- Maps and models — converting a scale distance to a real one.
- Resizing — keeping an image or drawing’s aspect ratio.
- Unit conversion — setting up a known rate as a ratio and solving.
Working with whole-number ratios and factors instead? See the GCF calculator, or compute a part of a whole with the percentage calculator.
Frequently asked questions
How do I solve a proportion?
A proportion says two ratios are equal: a/b = c/d. To find a missing value you cross-multiply, so a × d = b × c, then divide to isolate the unknown. For 3/4 = x/20, cross-multiply to 4x = 60, so x = 15. Leave one box blank above and the calculator solves it.
What is a proportion?
A proportion is a statement that two ratios are equal, such as 1/2 = 5/10. It is used to scale recipes, convert units, resize images and read maps — anywhere two quantities keep the same ratio. If three of the four parts are known, the fourth is fixed.
What is cross-multiplication?
Cross-multiplication is the shortcut for solving a proportion: multiply each numerator by the opposite denominator and set the products equal. For a/b = c/d it gives a·d = b·c. It works because multiplying both sides of the equation by b and d clears the fractions.