How to Calculate Percent Error
Percent error tells you how far an experimental or measured value deviates from a known or theoretical value, expressed as a percentage. It's a standard way to evaluate the accuracy of measurements in science labs and quality control.
The Formula
Percent Error = |Measured − Accepted| / |Accepted| × 100%
The absolute value ensures percent error is always positive (some formulas include the sign to indicate over or under-measurement).
Step-by-Step Example
In a lab, you measure the boiling point of water and get 98.5°C. The accepted value is 100°C.
Percent Error = |98.5 − 100| / |100| × 100% = 1.5 / 100 × 100% = 1.5%
Percent Error vs. Percent Difference
These are frequently confused:
| Metric | Formula | Use When |
|---|---|---|
| Percent Error | |measured − accepted| / accepted | Comparing to a known standard |
| Percent Difference | |value1 − value2| / average × 100 | Comparing two experimental values |
Acceptable Percent Error
What counts as "acceptable" depends on the context:
- Physics/chemistry lab: < 5% is typical; < 1% for careful work
- Engineering tolerances: < 0.1–1% depending on application
- Survey data: ± 3–5% at 95% confidence is standard
Types of Experimental Error
Systematic error: Consistent bias in one direction (e.g., a scale that reads 0.3 g too high). Affects accuracy, not precision. Reduces percent error closer to zero with better calibration.
Random error: Unpredictable variation around the true value. Affects precision. Reduced by taking multiple measurements and averaging.
Use our percent error calculator for any experimental measurement.