Percent Error Calculator
How far a measured or estimated value is from the true one, as a percentage.
Good to know. A 1% error sounds tiny, until the measurement is the dose of something.
How it works
In plain words
Take the difference between the measured value and the true value, ignore its sign, divide by the true value, and multiply by 100.
The formula
percent error = |measured − true| ÷ |true| × 100
Worked example
Measuring gravity as 9.6 m/s² against the accepted 9.81: |9.6 − 9.81| = 0.21, and 0.21 ÷ 9.81 × 100 = 2.1407%.
Sources
No outside data or rates: this is plain arithmetic.
3 automated tests check its answers on every change.
About the percent error calculator
Percent error tells you how close a measurement or estimate came to the true value, as a share of that value. It is a standard part of school and university science labs, and useful anywhere you compare a result with a known figure: a scale against a reference weight, an estimate against the final cost, a forecast against what happened.
Subtract the true value from the measured one, drop the sign, and divide by the true value. The result is always positive; the calculator also shows whether the measurement was too high or too low, and the signed error, which some courses use.
Dividing by the true value is what makes it percent error rather than percentage difference. When neither value is the “right” one, for example two measurements of the same thing, use the percentage difference calculator, which divides by their average instead.
What counts as a good result depends on the experiment. In a school lab, under 5% is often considered good. In precise work, like calibrating instruments, errors are measured in fractions of a percent. The percent error only measures accuracy, how close you were to the true value, not precision, how close repeated measurements are to each other.
Questions people ask
How do I calculate percent error?
Divide the difference between the measured and true value by the true value, ignore the sign, and multiply by 100.
Can percent error be negative?
Usually it is given as a positive number. The signed version is negative when the measurement is below the true value.
What is the difference between percent error and percent difference?
Percent error compares with a known true value. Percent difference compares two values when neither is the reference, dividing by their average.