Percent Error Calculator
Find the percent error between a measured or estimated value and the accepted true value: |measured − true| ÷ |true| × 100.
Last updated: October 2026
Percent Error
2.14 %
Formula below · 1 source (mathsisfun.com) · Updated Oct 2026
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About this calculator
Percent error measures how far a measurement or estimate is from the accepted (true) value, as a percentage of the true value:
percent error = |measured − true| ÷ |true| × 100
With the defaults, a pendulum experiment gives g = 9.6 m/s² against the accepted 9.81 m/s²: |9.6 − 9.81| ÷ 9.81 × 100 = 2.14%.
Why the absolute value
The bars make the result positive whether you measured too high or too low, so percent error describes the size of the miss. If you need the direction, use the signed version (measured − true) ÷ true × 100: here it would be −2.14%, an underestimate. Some lab manuals use the signed form, so check which one your course expects.
Limits
Percent error needs a known true value that is not zero. To compare two measurements when neither is the reference, use percent difference instead, which divides by their average. A small percent error means the result is close to the accepted value (accuracy); it says nothing about how repeatable the measurement was (precision).
| Measure | Formula | Result |
|---|---|---|
| Percent error | |9.6 − 9.81| ÷ 9.81 × 100 | 2.14% |
| Signed percent error | (9.6 − 9.81) ÷ 9.81 × 100 | −2.14% |
| Percent difference | |9.6 − 9.81| ÷ ((9.6 + 9.81) ÷ 2) × 100 | 2.16% |
How to use
Enter your measured or estimated value and the accepted value. Example 1: g measured as 9.6 m/s², accepted 9.81 m/s². |9.6 − 9.81| ÷ 9.81 × 100 = 2.14%. Example 2: a student measures the boiling point of ethanol as 76 °C; the accepted value is 78.4 °C. |76 − 78.4| ÷ 78.4 × 100 = 3.06%. Example 3: you estimate a room at 12 m² and it measures 15 m². |12 − 15| ÷ 15 × 100 = 20%.
Frequently asked questions
How do you calculate percent error?
Subtract the true value from the measured value, drop the sign, divide by the true value, and multiply by 100. Measured 52, true 50: |52 − 50| ÷ 50 × 100 = 4%.
What is a good percent error?
It depends on the experiment and the equipment. In many introductory labs a few percent is reasonable, while precision measurements aim far lower. Compare your result with the uncertainty of your instruments rather than a single cut-off.
Can percent error be negative?
The standard formula uses the absolute value, so it is never negative. The signed form, (measured − true) ÷ true × 100, is negative when the measurement is too low; some textbooks use it to show the direction of the error.
What is the difference between percent error and percent difference?
Percent error compares a value with a known true value and divides by that true value. Percent difference compares two values with no reference and divides by their average: for 9.6 and 9.81 it is 0.21 ÷ 9.705 × 100 = 2.16%.
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