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Math calculator

Proportion Calculator

Solve the missing value in a proportion using cross multiplication.

✓ Tested formula✓ Instant result✓ Private by design
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Enter your valuesResults update instantly as you type.

✓ Your numbers stay on this device.

Understand the answer

Formula and method

FormulaIf A/B = C/X, then X = B × C ÷ A.

Cross multiplication converts two equal ratios into A × X = B × C, making the unknown easy to isolate.

Worked example

3/5 = 12/x gives x = 5 × 12 ÷ 3 = 20.

How to use this calculator

  1. Enter the values using the labels and units shown.
  2. Review the live result and supporting details.
  3. Check the formula, example and assumptions before using the answer.

Assumptions and limits

  • A must not be zero.

Reference

Formula reviewed 2026-07-15

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