Percent Error Calculator
Calculate the percent error between an experimental (observed) value and a theoretical (expected) value. Used in lab work, surveys and quality control.
How to use the Percent Error Calculator
- Enter your inputs into the Percent Error Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The percent error formula
Percent error = |Observed − Expected| / |Expected| × 100
The absolute value of the difference between observed and expected, divided by the expected value, expressed as a percentage. The version without absolute value gives signed error (positive or negative).
Worked example
A lab measurement gives 9.7 m/s² for gravitational acceleration; expected value is 9.81 m/s². Percent error = |9.7 − 9.81|/|9.81| × 100 = 1.12%. The signed error is −1.12% (observed below expected).
Frequently asked questions
What is acceptable percent error?
Depends on the field. Physics lab work commonly targets <5%. Manufacturing tolerances may demand <0.1%. Survey research often accepts wider margins like ±3 percentage points.
When should I use signed vs. absolute error?
Use absolute when only magnitude matters. Use signed when the direction (high vs. low) carries information (e.g., calibration drift, systematic bias).
How is this different from percent difference?
Percent error compares to a "true" or expected value. Percent difference compares two equally-valid measurements: |A − B| / ((A+B)/2) × 100. Neither is inherently better — choose based on whether a reference value exists.