A walkthrough, end to end.
- 1
Enter the experimental (measured) value and the theoretical (true / accepted) value.
- 2
The calculator returns the absolute percent error.
- 3
Use for lab reports, measurements, and validating experimental procedures.
Percent error formula
Percent error = | experimental − theoretical | / | theoretical | × 100. The absolute value bars ensure a positive result; some textbooks use signed percent error to indicate direction (above or below true value).
What you can do with this.
Chemistry lab reports
Measured 9.85 g, expected 9.81 g. % error = |9.85 − 9.81| / 9.81 × 100 = 0.41%. Most intro chemistry labs accept under 5% error; precision chemistry under 1%.
Physics measurements
Measured speed of sound 342 m/s vs. theoretical 343 m/s at room temperature: |342 − 343|/343 × 100 = 0.29% error. Tight measurements like this validate your apparatus.
Engineering tolerance
Manufactured part 25.04 mm vs. spec 25.00 mm: 0.16% error. Compare against tolerance specification (e.g., ±0.1% might fail; ±0.5% would pass).
Forecasting accuracy
Predicted sales $115K, actual $122K: % error = 5.7%. Lower percent error = better forecast model. Useful for evaluating predictive analytics.
Calibration check
Reference weight 100.00 g, scale reads 100.05 g: 0.05% error. If above acceptable threshold, recalibrate or replace the scale.
Signed vs. absolute percent error
This calculator gives absolute (always positive). For 'experimental was 5% TOO HIGH' direction, manually note the sign of (experimental − theoretical) before applying absolute value.
When percent error fails
When theoretical = 0, percent error is undefined (divide by zero). For near-zero theoretical values, use absolute error instead, or relative error against a different reference.
Percent error 2026 — what's current
Modern lab software auto-computes percent error from instrument-uploaded data. Standalone calculators remain useful for one-off calculations and verification.
Frequently asked.
Discipline-dependent. Intro labs: under 5%. Quantitative chemistry: under 1%. Engineering: depends on tolerance spec. Physics theory tests: often under 0.1%. Compare against your assignment's stated threshold.
Percent error compares experimental against a known true value. Percent difference compares two experimental values without a 'true' reference. Different formulas, different uses.
Convention — percent error is reported as a positive magnitude regardless of direction. If you need to know the experimental was above or below true, note that separately.
No. Calculations run entirely in your browser.