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.
結果
50
X (A/B = C/X)
How this was calculated
- 1Cross-multiply: B × C5 × 20 = 100
- 2Divide by A÷ 2 = 50
公式
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.
ステップ解説
- 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.
計算例
2/5 = 20/X
→ X = (5 × 20) ÷ 2 = 50.
A recipe for 4 people uses 300 g flour; how much for 6 people?
→ A=4, B=300, C=6 → X = (300 × 6) ÷ 4 = 450 g.
A 1:50 scale drawing has an 8 cm wall; how long is it in reality?
→ A=1, B=50, C=8 → X = (50 × 8) ÷ 1 = 400 cm (4 metres).
Reference table
| 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.
よくある間違い
- 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.
活用シーン
- 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.
よくある質問
How is this different from a percentage?
A percentage is a special case of a proportion where B is always 100; this calculator works with any B.
What happens if A equals 0?
The calculator returns no result, because division by zero is not mathematically defined. Choose an A that is not zero.
Can I use this to convert a scale ratio too?
Yes. Enter the scale as A/B (for example 1/50) and the measured length on the drawing as C; the calculator returns the real-world length as X.
Does this work with negative numbers too?
Yes, the formula X = (B × C) ÷ A also works mathematically with negative values, though these are rare in practice for lengths or quantities.
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