Proportion Calculator
Solve A : B = C : D for any missing value using cross multiplication.
What the Proportion Calculator does
Two ratios are in proportion when their cross products match: A × D = B × C. Knowing any three terms determines the fourth, which is how recipe scaling, unit conversion and map distances all work.
Formula
A : B = C : D → A × D = B × C
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| A | number | Optional | — |
| B | number | Optional | — |
| C | number | Optional | — |
| D | number | Optional | Fill in exactly three of the four boxes. |
How to use it
- Optionally add A, B and C.
- Select Calculate.
Worked example
A recipe for 4 people needs 300 g of rice. How much for 7 people?
- A
- 4
- B
- 300
- C
- 7
- D
- (blank)
D = (300 × 7) ÷ 4 = 525 g.
Reading the result
- Two ratios are in proportion when their cross products match, so knowing any three terms fixes the fourth.
- This is the arithmetic behind recipe scaling, map distances and unit conversion alike.
Common mistakes
- Crossing the terms in the wrong direction, which inverts the answer.
- Applying direct proportion to an inverse relationship. If doubling one quantity halves the other, the product stays constant rather than the ratio.
Frequently asked questions
Does this work for inverse proportion?
No. This solves direct proportion. For inverse relationships (more workers, less time) use A × B = C × D instead.