Proportion Calculator
Solve proportions: A:B = C:D. Enter three values and leave one blank to find the missing value. The calculator uses cross-multiplication: if A/B = C/D, then A*D = B*C. For example, if 2/3 = 4/?, the missing value is 6.
Cross-multiplication formula
If A/B = C/D, then A*D = B*C
Solving for D: D = (B*C) / A
Examples
| Proportion | Missing value | Solution |
|---|---|---|
| 2/3 = ?/6 | 4 | (3*2)/6 = 1? No, 2*6/3 = 4 |
| 1/2 = 3/? | 6 | (2*3)/1 = 6 |
| 5/8 = 10/? | 16 | (8*10)/5 = 16 |
Proportion calculator: frequently asked questions
What is a proportion?
A proportion is an equation stating that two ratios are equal. For example, 2/3 = 4/6, or written as a ratio, 2:3 = 4:6. Proportions are used to solve problems involving scaling and ratios.
How do I solve a proportion?
Use cross-multiplication. If A/B = C/D, then A*D = B*C. For example, if 2/3 = ?/6, cross-multiply: 2*6 = 3*?, so 12 = 3*?, so ? = 4.
What can I use proportions for?
Proportions are used for scaling recipes, maps, converting units, solving word problems, and finding missing values in similar figures.
Can I leave a proportion unsimplified?
Yes. A proportion like 2:3 = 4:6 is valid even though both sides can simplify to 2:3. Proportions are useful as written.
What if two values form the same ratio?
If the left and right sides have the same simplified ratio, the proportion is true. For example, 6:9 = 4:6 is false because 6:9 simplifies to 2:3, while 4:6 simplifies to 2:3. Actually, they are equal, so it is true.
Official sources
- Cross-multiplication method for solving proportions (standard mathematical formula).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.