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Proportion calculator

Find the missing number in a proportion such as 1 : 5 = ? : 10.

Enter the proportion

Your data stays in this browser.

Enter any three numbers. Leave the number you want to find empty.

Example: 1 : 5 = □ : 10

Result

Enter three numbers above. The calculator fills the empty box.

How to use the proportion calculator

  • Write the proportion in the four boxes: a : b = c : d.
  • Enter three numbers and leave the number you want to find empty.
  • Select Calculate to fill the empty box.

Example

For 1 : 5 = ? : 10, enter 1, 5, and 10. The missing value is 1 × 10 ÷ 5 = 2.

Input and limits

  • Use ordinary decimals with a point and no units or separators.
  • Exactly one box must be empty.
  • A calculation that requires division by zero has no single answer.
  • Displayed results use up to two decimal places; very small or large values use scientific notation.