Percent Error Calculator guide
Percent Error Calculator: Measured vs True Values
Use our percent error calculator to work out how far a measured or experimental value is from the true value, as a percentage.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
In science experiments, your results are rarely exactly right. Percent error tells you how close you got. Our Percent Error Calculator works it out.
How to use the calculator (Percent Error Calculator)
- Enter your measured (experimental) value.
- Enter the true (accepted or theoretical) value.
You'll see the percent error, the absolute and relative errors, and whether your value was too high or too low.
The formula
Percent error = |measured − true| ÷ |true| × 100
The straight lines mean "ignore the minus sign", so percent error is always positive. The signed version, shown too, tells you whether you were over or under.
A worked example
With the calculator's default settings, you measure the acceleration due to gravity as 9.6m/s². The accepted value is 9.81m/s².
- Absolute error: |9.6 − 9.81| = 0.21
- Percent error: 0.21 ÷ 9.81 × 100 = 2.14%
- Your result was too low.
What's a good percent error?
It depends on the experiment. In school science, under 5% is often good, and under 1% is excellent. In precise lab work, errors may need to be far smaller.
Frequently asked questions
Is percent error the same as percentage change?
They use a similar formula, but percentage change compares a new value to an old one. For that, use our Percent Change Calculator.
What if the true value is zero?
Percent error can't be worked out, because you'd divide by zero. Use the absolute error instead.
How do I reduce errors?
Repeat measurements and take an average, use more precise equipment and check for systematic errors. Our Error Propagation Calculator shows how uncertainties combine.
Sources and further reading
- Wolfram MathWorld: Percentage errormathworld.wolfram.com
- NPL: Good practice guides on measurementnpl.co.uk
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This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.