Percent Error Calculator
How far a measurement is from the accepted value.
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Formula
Percent error = |experimental − accepted| ÷ |accepted| × 100
The difference between what you measured and the accepted value, divided by the accepted value, as a percentage. The absolute value makes it positive whichever way you missed; the signed figure underneath keeps the direction, which lab reports often ask about.
Worked example
Measuring gravitational acceleration as 9.6 against an accepted 9.8 is off by 0.2. 0.2 ÷ 9.8 × 100 = 2.0408%, and because the measurement came in under, the signed error is −2.0408%.
Where this can give the wrong answer
- The accepted value is always the denominator. Dividing by your own measurement instead gives a different number and is the most common mistake in lab write-ups.
- An accepted value of zero makes percent error undefined — report the absolute error instead.
- Percent error compares against a known value. When comparing two measurements with no accepted value, the right tool is percent difference, which divides by their average.
FAQ
- The standard formula takes the absolute value, so no. Some instructors want the sign to show whether you were high or low; that is the signed error shown beneath the headline.
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