Percent Error Calculator
Calculate percent error between a measured and true value, with absolute error and signed result.
Inputs
The value obtained from experiment or measurement.
The known or reference value.
Percent Error (signed)
-0.1019%
Positive means measured > actual; negative means measured < actual.
Percent Error (unsigned)
0.1019%
The absolute value form — use this when your course asks for |% error|.
Absolute Error
-0.010000
Measured − Actual, in the same units as your input.
Relative Error (decimal)
-0.001019
Step by step
Absolute error
Measured − Actual = 9.8 − 9.81
= -0.01
Divide by accepted value
-0.01 ÷ 9.81
= -0.001019
Convert to percentage
-0.001019 × 100
= -0.1019%
Absolute percent error (unsigned)
|-0.1019%|
= 0.1019%
How it works
Percent error quantifies the accuracy of a measurement by expressing how far it deviates from the accepted value as a fraction of that value. A small percent error means the measurement is close to the true value. The sign tells you the direction of the error: positive means you overestimated, negative means you underestimated.
Formulas
Signed percent error
% Error = (Measured − Actual) / |Actual| × 100
- V_measured
- Experimentally measured value
- V_actual
- Accepted (true) reference value
Absolute error
Absolute Error = Measured − Actual
Frequently Asked Questions
What is a good percent error?
It depends on the field. In most high-school lab work, an error below 5% is considered acceptable. In analytical chemistry, 1% or below is typical. In particle physics, errors below 0.01% are expected. Always check the tolerance specified by your experiment or industry standard.
Should I use the signed or unsigned result?
Use the signed result (which can be negative) when the direction matters — overshooting versus undershooting the true value are different kinds of systematic error. Use the unsigned (absolute) form when you only care about the magnitude, which is what most textbook problems ask for.
Why is percent error undefined when the actual value is zero?
Percent error divides by the actual value. Division by zero has no mathematical definition. If the accepted value is zero, you can report the absolute error instead, but a percentage has no meaning.
How is percent error different from percent difference?
Percent error compares a measurement to a known accepted value. Percent difference compares two measurements of equal standing when neither is the 'truth', dividing by their average instead of one specific value.