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Proportion Calculator

A proportion like A/B = C/X comes up in scale drawings, recipes and conversions. Enter A, B and C to find the missing X.

Result

50

X (A/B = C/X)

How this was calculated
  1. 1Cross-multiply: B ร— C5 ร— 20 = 100
  2. 2Divide by Aรท 2 = 50

Formula

X = (B ร— C) รท A

This follows from cross-multiplication: from A/B = C/X it follows that A ร— X = B ร— C, so X = (B ร— C) รท A. This method works for any proportion with one unknown.

Worked examples

2/5 = 20/X

โ†’ X = (5 ร— 20) รท 2 = 50.

A recipe for 4 people uses 300 g flour; how much for 6 people?

โ†’ A=4, B=300, C=6 โ†’ X = (300 ร— 6) รท 4 = 450 g.

Common mistakes

  • Placing A and C, or B and X, in the wrong position, which inverts the result.

When to use it

  • Scaling a recipe up or down for a different number of people.
  • Converting a scale drawing to real-world dimensions.

Frequently asked questions

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.

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