Percent Error Calculator
Compare a measured value against the true value as a percent error.
About Percent Error Calculator
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
Worked example
With the default values below, Percent Error Calculator returns Percent Error (signed) of -0.1019%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Measured (experimental) value
- 9.8
- Accepted (true) value
- 9.81
Results
- Percent Error (signed)Positive means measured > actual; negative means measured < actual.
- -0.1019%
- Percent Error (unsigned)The absolute value form — use this when your course asks for |% error|.
- 0.1019%
- Absolute ErrorMeasured − Actual, in the same units as your input.
- -0.010000
- Relative Error (decimal)
- -0.001019
Step by step
- 1Absolute error= -0.01
- 2Divide by the magnitude of the accepted value= -0.001019
- 3Convert to percentage= -0.1019%
- 4Absolute percent error (unsigned)= 0.1019%
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.