Formula
How percent error is calculated
Percent error =|experimental value − accepted value| ÷ |accepted value| × 100
First find the absolute difference between the experimental and accepted values. Divide that difference by the absolute accepted value, then multiply the result by 100.
Worked example
Percent error example
A student measures a substance at 9.8 grams. The accepted value is 10 grams.
- Find the difference: |9.8 − 10| = 0.2
- Divide by the accepted value: 0.2 ÷ 10 = 0.02
- Multiply by 100: 0.02 × 100 = 2%
The experimental measurement therefore has a percent error of 2%.
Laboratory guidance
Common percent-error mistakes
- Dividing by the experimental value instead of the accepted value.
- Forgetting to use the absolute value of the difference.
- Forgetting to multiply the decimal result by 100.
- Mixing values that use different measurement units.
- Rounding too early during the calculation.
Related data analysis
Compare and evaluate experimental results
Use the Percent Difference Calculator when comparing two experimental values without a single accepted reference value.
Use the Measurement Uncertainty Calculator to assess the precision and reliability of measured laboratory values.
Interpretation limits
Assumptions and limitations
Percent error assumes that a meaningful accepted or reference value is available. The experimental and accepted values must represent the same quantity and use compatible units.
A small percent error does not prove that a method is precise, and a large value does not identify the source of error. The calculation also becomes undefined when the accepted value equals zero.
Questions and answers
Percent error FAQ
What is percent error?
Percent error measures how far an experimental value is from an accepted or theoretical value, expressed as a percentage of the accepted value.
Can percent error be negative?
Percent error is normally reported as a non-negative value because the difference is converted to an absolute value before division.
Why can the accepted value not be zero?
The accepted value is used as the denominator. Dividing by zero is undefined, so the standard percent-error formula cannot be used when the accepted value is zero.