Percent error tells you how close a measurement is to the correct answer: |measured − true| / |true| × 100. The measured (experimental) value is what you obtained; the true (accepted) value is the known reference. A small percent error means high accuracy. This tool also reports the signed error so you can see whether you read high or low.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the percent error definition, recomputed in code.
The percent error formula
Subtract the true value from the measured value to get the error. Divide by the true value and multiply by 100 for the percentage; taking the absolute value gives the standard (always-positive) percent error. The signed version keeps the sign — positive when the measurement is too high, negative when too low — which is useful for spotting a systematic bias.
Worked examples
Measuring g as 9.8 when the accepted value is 9.81:
A reading of 102 against a true 100:
A reading of 48 against a true 50:
So measuring gravity as 9.8 is extremely accurate (0.10%), reading 102 is 2% high, and reading 48 is 4% low. The signed error flags the direction; the percent error flags the size.
Frequently Asked Questions
|measured − true| / |true| × 100. Measured 102, true 100 → 2/100 × 100 = 2%.
The known correct reference — a constant, standard or accepted figure. The measured value is what you got.
Standard percent error uses absolute value (positive). The signed error is negative if you read low, positive if high.
Smaller is better. A few percent may be fine in a teaching lab; precision work wants a fraction of a percent.
Percent error needs a true value. Percent difference compares two values with no reference; percent change is over time.