Proportion Calculator
Solve a proportion (A:B = C:D) for the missing fourth value.
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- Accurate results
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- Works on all devices
Enter three known values in a proportion to solve for the fourth.
Your result
15
D
AI explanation
Formula
A:B = C:D, so D = (B × C) ÷ A (cross-multiplication)Worked example
2:3 = 10:D
| Field | Value |
|---|---|
| A | 2 |
| B | 3 |
| C | 10 |
| D | 15 |
Assumptions
- Solves the proportion A:B = C:D for D specifically — rearrange your values if you need to solve for a different position.
Frequently asked questions
What is cross-multiplication?
For a proportion A:B = C:D, cross-multiplication gives A × D = B × C, which rearranges to D = (B × C) ÷ A.
What if I need to solve for A, B, or C instead of D?
Rearrange your known and unknown values so the missing one is in the "D" position — for example, to solve for A given B, C, D, treat it as B:A = D:C and enter accordingly.
What are proportions used for?
Scaling recipes, converting units, map scales, and comparing rates all commonly use proportions.