Calculate the percent error between a measured (observed) value and the true (actual) value. Useful in science, engineering, and quality control to assess measurement accuracy.
Error between 9.8 and 9.81
0.1019367992%
|9.8 − 9.81| ÷ |9.81| × 100 = 0.1019367992%
Absolute Error0.01
Measured Value9.8
True Value9.81
Tip: The calculator updates instantly as you type. Percent error is always positive.
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What is the Percent Error Calculator?
Percent error quantifies the difference between an observed (measured) value and a true (actual) value as a percentage. It answers: 'How far off was my measurement?' A 2% error means the measurement was off by 2% of the true value. Percent error is always positive because it measures magnitude of error, not direction. Common in physics experiments, quality control, calibration, and anywhere accuracy matters.
How it works
Enter your measured value and the true value. The calculator finds the absolute difference between them, divides by the absolute true value, and multiplies by 100 to get the percentage. For example, if you measured 9.8 m/s for gravity but the true value is 9.81 m/s, the percent error is |9.8 − 9.81| ÷ |9.81| × 100 = 0.102%. The tool displays the percent error, absolute error, and both input values for reference.
Percent Error = |Observed − True| ÷ |True| × 100
Observed is the measured or experimental value. True is the accepted or theoretical value. The vertical bars | | mean absolute value (always positive). Subtract to find the difference, divide by the true value's magnitude, then multiply by 100 to express as a percentage.
Examples
Input
Result
Notes
Measured gravity as 9.8 m/s², true value 9.81 m/s²
0.102%
|9.8 − 9.81| ÷ |9.81| × 100 = 0.102%. Very accurate measurement, typical in physics labs.
Measured liquid volume as 95 mL, true volume 100 mL
5%
|95 − 100| ÷ |100| × 100 = 5%. Small but noticeable error, common in chemistry.
Predicted attendance 150 people, actual attendance 162 people
7.41%
|150 − 162| ÷ |162| × 100 = 7.41%. Moderate error in event planning or forecasting.
How to use the Percent Error Calculator
Enter your measured (observed) value—the number you recorded or calculated experimentally.
Enter the true (actual) value—the accepted, theoretical, or reference value.
The calculator instantly shows the percent error as a positive percentage.
Check the absolute error to see the raw difference between the two values.
For repeated measurements, adjust inputs and recalculate to compare different trials.
Benefits
Quantifies accuracy in one number—eliminates guesswork about whether a measurement is 'close enough.'
Essential for validating experimental results and identifying systematic or random errors in data collection.
Helps engineers and technicians verify calibration and identify when equipment needs adjustment or repair.
Supports quality control in manufacturing and science by flagging deviations beyond acceptable thresholds.
Free, instant, and works offline—no subscriptions or software installation needed.
Shows both percent and absolute error, so you understand both relative and real-world magnitude of deviation.
Tips & common mistakes
Common mistakes
Forgetting to use absolute value—the difference can be negative depending on whether measured is above or below true, but percent error must be positive.
Dividing by the measured value instead of the true value—the formula always divides by the true/reference value, not the observed one.
Confusing percent error with percent change—percent change tracks direction (up or down); percent error only measures magnitude of inaccuracy.
Leaving the true value as zero—if true value is zero, percent error is undefined (the calculator will flag this).
Tips
Compare percent errors across multiple trials to identify whether errors are random (scatter) or systematic (all too high or too low).
In labs, percent error below 5% is often considered good; below 2% is excellent; above 10% warrants investigation.
Use absolute error when the unit matters (e.g., 'We were off by 0.5 kg'). Use percent error for comparisons across different scales or units.
Record both values carefully—even a small typo in the true value will drastically change the calculated error.
If you have multiple measurements, calculate percent error for each and then average them to get a sense of overall accuracy.
Frequently asked questions
Why is percent error always positive?
The formula uses absolute value (|...|), which removes the sign. Percent error measures how far off you were, not whether you overestimated or underestimated. Use the raw difference if direction matters.
What's the difference between percent error and percent difference?
Percent error compares a measured value to an accepted true value. Percent difference compares two experimental values with no 'true' reference. Use percent error when one value is the standard.
Is a high percent error always bad?
Context matters. A 0.1% error in measuring gravity is excellent; a 10% error in that same context is terrible. In other fields (like survey estimates), 5% error might be acceptable.
Can percent error ever be over 100%?
Yes. If you measure 250 when the true value is 100, percent error is 150%. This indicates you were off by more than the true value itself—usually a sign of major measurement error or wrong units.
How do I reduce percent error in experiments?
Use better instruments, calibrate equipment before use, control environmental variables, take multiple trials and average, and follow procedures carefully. Systematic errors (like a miscalibrated scale) persist; random errors average out.
Should I use this for comparing two measurements with no known true value?
No—use a percent difference calculator instead. Percent error requires a reference value you accept as true or theoretical. If both measurements are experimental estimates, percent difference is more appropriate.