⚖️ Proportion Calculator
A proportion like A/B = C/X shows up everywhere: scale drawings, recipes you want to scale, mixing ratios and unit conversions. Enter A, B and C and this calculator instantly finds the missing fourth term X, showing the cross-multiplication steps.
X (A/B = C/X)
50
Cross-multiply: B × C: 5 × 20 = 100
Divide by A: ÷ 2 = 50
Formula
X = (B × C) ÷ A
This follows from cross-multiplication: from A/B = C/X it follows that A × X = B × C, so X = (B × C) ÷ A. This method works for any proportion with exactly one unknown, whichever of the four positions it occupies.
Steps
- 1Enter the three known values A, B and C.
- 2Cross-multiply: B × C, then divide by A.
- 3Read off the result X, including the intermediate steps.
Proportions solved for X
| A | B | C | X = (B×C)÷A |
|---|---|---|---|
| 1 | 2 | 10 | 20 |
| 2 | 5 | 20 | 50 |
| 4 | 300 | 6 | 450 |
| 5 | 8 | 25 | 40 |
| 10 | 3 | 50 | 15 |
Examples for a quick check; enter your own A, B and C for your own X.
commonMistakes
- Placing A and C, or B and X, in the wrong position, which inverts the result.
- Forgetting that A cannot be zero, since division by zero is undefined.
- Not keeping the units of B and X the same, making the answer incomparable with the original.
useCases
- Scaling a recipe up or down for a different number of people.
- Converting a scale drawing or blueprint to real-world dimensions.
- Working out mixing ratios, such as paint, concrete or chemical solutions.