Proportion Calculator
This proportion calculator solves the missing value in A : B = C : D when you enter any three numbers. A proportion is solved by cross multiplication: the product of one diagonal equals the product of the other, so the unknown equals the opposite diagonal product divided by the remaining known value.
Solve a proportion
Preset: 3 : 4 = x : 20, so x = 15.
Worked example: 3 : 4 = x : 20
Cross multiplication says the diagonal products must match. For 3 : 4 = x : 20, the known diagonal is 3 x 20, and the remaining known value is 4.
- Write the cross products: 3 x 20 = 4x.
- Isolate the missing value: x = 3 x 20 / 4.
- Compute the result: x = 60 / 4 = 15.
- So x = 15. Check: 15 : 20 simplifies to 3 : 4.
Common proportion problems
| Proportion | Missing value | Answer |
|---|---|---|
| 3 : 4 = x : 20 | x | 15 |
| 2 : 7 = x : 35 | x | 10 |
| 5 : 8 = 15 : x | x | 24 |
| 9 : 12 = 15 : x | x | 20 |
| x : 6 = 14 : 21 | x | 4 |
| 4 : x = 12 : 18 | x | 6 |
| 0.5 : 2 = 3 : x | x | 12 |
| 7 : 10 = x : 50 | x | 35 |
| 12 : 5 = x : 20 | x | 48 |
| 1.5 : 4 = 6 : x | x | 16 |
| x : 9 = 8 : 12 | x | 6 |
Related ratio tools and guides
Use the homepage ratio solver when you also need to simplify or scale a ratio. Convert a ratio into a fraction with the ratio to fraction converter, solve screen dimensions with the aspect ratio calculator, or read the cross multiplication guide for a slower walkthrough.
Frequently asked questions
What is a proportion calculator?
A proportion calculator finds the missing value in two equal ratios, usually written as A : B = C : D. Enter any three values and the calculator solves the fourth value with cross multiplication.
How do you solve a proportion by cross multiplication?
Multiply the two known numbers on one diagonal, then divide by the remaining known number. For A : B = C : D, the rule is A times D equals B times C.
Can any of the four values be missing?
Yes. The missing value can be A, B, C, or D. The calculator checks which box is blank and uses the correct cross multiplication formula for that position.
Can a proportion include decimals?
Yes. Decimals work the same way as whole numbers. For example, 1.5 : 4 = 6 : x gives x = 16 because 4 times 6 divided by 1.5 equals 16.
What happens if a value is zero?
Zero can appear in a proportion, but the calculator cannot divide by zero. If the missing value would require division by zero, the tool explains the problem instead of showing an invalid result.
Is my proportion data stored?
No. The calculation runs locally in your browser with plain JavaScript. Nothing you type is sent to a server or saved.