Solve a proportion
Proportion Calculator
Enter any three values in two equal ratios and leave the value you want to find blank. The calculator solves the missing value and shows the cross-multiplication working.
Solve for the missing value
Use the form a/b = c/d. Leave exactly one box blank.
Missing value
Example: 3/5 = x/20
Cross multiply: 3 × 20 = 5 × x. That gives 60 = 5x, so x = 60 ÷ 5 = 12.
What is a proportion?
A proportion states that two ratios are equal. For example, 3:5 and 12:20 are equivalent ratios, so 3/5 = 12/20 is a true proportion.
How to solve proportions
For a proportion written as a/b = c/d, multiply diagonally to create the equation a × d = b × c. Then divide by the number next to the missing value.
Proportion examples
| Proportion | Missing value |
|---|---|
| 3/5 = x/20 | x = 12 |
| x/8 = 6/12 | x = 4 |
| 4/x = 10/15 | x = 6 |
| 2/3 = 8/x | x = 12 |
Proportion calculator FAQs
How do you solve a proportion?
For a/b = c/d, cross multiply to get a × d = b × c, then divide to isolate the missing value.
What is the proportion formula?
A proportion can be written as a/b = c/d. If the two ratios are equal, their cross products are equal: a × d = b × c.
Can I use fractions and decimals?
Yes. The calculator accepts whole numbers, decimals and fractions such as 3/4.
How many values should I enter?
Enter exactly three values and leave the value you want to find blank.