Percent Error Calculator
Calculate the percent error between a measured or experimental value and the true or accepted value.
The percent error formula
Percent error tells you how far a measurement is from the accepted value, as a percentage of that value. It is used throughout science labs, engineering and quality control.
Percent error = |Measured − True| ÷ |True| × 100
In a physics lab you measure gravitational acceleration as 9.6 m/s². The accepted value is 9.81 m/s². Percent error = |9.6 − 9.81| ÷ 9.81 × 100 = 2.14%.
Signed vs absolute error
The standard definition uses the absolute value, so percent error is always positive. Some fields keep the sign to show direction: a negative result means the measurement was too low, a positive one too high. The calculator shows both.
Percent error vs percent difference
Use percent error when one value is known to be correct. Use percent difference, found in the percentage calculator, when comparing two measurements where neither is the reference; it divides by their average instead.
What is a good percent error?
It depends on the field. In school labs, under 5% is usually considered good and under 1% excellent. Precision manufacturing may demand tolerances of a fraction of a percent. Sources of error include instrument limits, reading errors, environmental conditions and simplifying assumptions in the method.
Frequently asked questions
Can percent error be negative?
With the standard formula it is always positive. If you keep the sign, negative means the measurement was below the true value.
What if the true value is zero?
Percent error is undefined when the true value is zero, because you cannot divide by zero. Report the absolute error instead.
Is percent error the same as accuracy?
Percent error measures accuracy, how close a result is to the true value. Precision describes how close repeated measurements are to each other.
Last reviewed Oct 20, 2026. Results are estimates; check important decisions with a qualified professional.