How far off was the measurement?
Percent error expresses the gap between a measured or estimated value and the true value as a share of the true value:
percent error = |measured − true| ÷ |true| × 100%
It is the standard way to report accuracy in science labs, and a handy way to judge estimates in business: a forecast of 52 units against an actual 50 is 4% off.
Worked example
A pendulum experiment gives g = 9.72 m/s², while standard gravity is 9.80665 m/s². The error is |9.72 − 9.80665| ÷ 9.80665 × 100 ≈ 0.88%. The signed error, −0.88%, shows the measurement came out low.
Percent error vs percent difference
When neither value is the "true" one (two measurements, two methods), percent difference divides by their average instead. This calculator uses the true value.
How this works
The page subtracts the true value from the measured value, divides by the absolute true value and multiplies by 100. The headline shows the size of the error; the signed error keeps the direction (negative means the measurement is too low), and the absolute error is the plain difference in your units. The true value must not be zero. Everything runs in your browser.
Frequently asked
Can percent error be negative?
The usual definition uses the absolute value, so it is positive. The signed version shows whether you were above or below.
What is a good percent error?
It depends on the field: under 5% is often acceptable in school labs; precision manufacturing may require under 0.1%.
What if the true value is zero?
Percent error is undefined; report the absolute error instead.
How is percent difference different?
It compares two values without a reference, dividing by their average.
Sources
Reviewed Oct 17, 2026 · test vectors fixtures/calculators/math/percent-error-calculator/