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Percent Error Calculator

Choose percent error to compare a measured value with a true value, or percent difference to compare two values neither of which is the reference. Enter both numbers to see the result.

Your numbers

Result

How we calculate it

Percent error = |measured − true| ÷ |true| × 100. It shows how far a measurement is from the true value as a share of the true value.

Percent difference = |a − b| ÷ |(a + b) ÷ 2| × 100. It compares two values by their average, so the order of the two numbers does not matter. A measurement of 9.8 against a true value of 9.81 has a percent error of about 0.10%.

Assumptions and limits

  • The true value cannot be zero for percent error, and the two values cannot average to zero for percent difference.
  • The absolute error is always positive. The signed error is negative when your measurement is below the true value.
  • Some books define percent difference in other ways. This calculator uses the average of the two values.

Frequently asked questions

What is percent error?

Percent error is the size of the difference between a measured value and the true value, shown as a percent of the true value. It is often used in science labs.

What is the difference between percent error and percent difference?

Percent error compares one value against a known true value. Percent difference compares two values when neither is the reference, using their average as the base.

Can percent error be negative?

The usual definition uses the absolute value, so it is never negative. The signed error line shows whether you were above or below the true value.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: What do you want to find? = Percent error (measured value against a true value); Measured value, or first value = 9.8; True value, or second value = 9.81.

  • Percent error 0.1%
  • Absolute error 0.01
  • Signed error -0.01

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.