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Rule of three calculator

Solve a simple rule of three, direct or inverse, with the formula shown. Enter three values and get the fourth.

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How to use: Rule of three calculator

  1. Choose whether the proportion is direct (more A, more B) or inverse (more A, less B).
  2. Enter A, B and C: “if A corresponds to B, what does C correspond to?”.
  3. Read the value of X and the formula used.

What it accepts and what you get

Input
Three numbers (decimal comma or point).
Output
The fourth value, X, with the formula shown.
Where your data is processed
Your data is processed in your browser and is not sent to our servers.

Limitations and notes

  • Direct: X = B × C ÷ A. Inverse: X = A × B ÷ C.
  • The value you divide by cannot be 0.
  • Check that it really is a direct or inverse proportion: the calculation cannot know that.

Frequently asked questions

How do I do a simple rule of three?

Enter A, B and C: if A corresponds to B, the tool works out what C corresponds to. For direct proportion it multiplies B by C and divides by A.

When do I use the inverse rule of three?

When one quantity goes up as the other goes down, like more workers and fewer days. Then X = A × B ÷ C; pick it in the selector.

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