Percentage error expresses the size of a measurement error relative to an accepted or reference value. It is common in experiments, quality checks, and estimates.
Use the accepted value as the reference
Subtract the accepted value from the measured value, take the absolute value, divide by the accepted value, then multiply by 100.
Work through an example
If a measured value is 49 and the accepted value is 50, the absolute error is 1. Divide 1 by 50 and multiply by 100. The percentage error is 2%.
Keep the sign only when it matters
Standard percentage error uses the absolute difference, so it reports size rather than direction. If overestimation or underestimation matters, also report the signed error separately.
Do not use a zero reference
Percentage error is undefined when the accepted value is zero. Use absolute error or a domain-appropriate alternative instead.
A calculator is often the quickest way to check your mental math. The important part is using the right starting value.
Open a calculator →Common questions
Can percentage error be more than 100%?
Yes, when the absolute error is larger than the accepted value.
Is percentage error the same as percentage difference?
No. Percentage error uses an accepted reference value; percentage difference treats both values more equally.
Should I round the measured values first?
Keep their justified precision and round the final percentage appropriately.
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