FreeToolStation

Percent Error Calculator

How far a measurement is from the accepted value.

Math Calculators Free, no sign-up Runs in your browser

What your experiment gave.

The true or published value.

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Percent error
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Absolute error
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Relative error
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Accuracy
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Measurement was

About the Percent Error Calculator

Percent Error Calculator compares a measurement against the value it should have been. The difference is divided by the accepted value, never by the measurement, because the accepted value is the reference the error is being judged against.

By convention percent error is reported as a positive number: the question is usually how far off the measurement was, not in which direction. The sign option is there for the cases where direction matters, such as calibrating an instrument that reads consistently high, and the direction readout says which way it went either way.

Absolute and relative error are shown alongside because they answer different questions. Absolute error is in the units you measured in, which is what matters when a tolerance is specified in millimetres. Relative error is the same thing as a fraction, which is what matters when comparing precision across measurements of very different sizes.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

Looking for something this does not cover? FreeToolStation.com is a second collection of the same kind of everyday tools, and it is worth a look if percent error calculator is only part of what you are working on.

How to use the Percent Error Calculator

  1. Enter measured value and accepted valueDecimals are fine, and thousands separators are accepted and ignored.
  2. Read the percent error, absolute error and relative errorThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
  3. Copy or check the resultCopy the summary with the copy button, or read the formula with your own numbers in it.

Features

  • Shows percent error, absolute error, relative error, accuracy and measurement was together, so you are not running it twice
  • Prints the formula with your own figures substituted in
  • Updates as you type, with no button to press and no page reload
  • Copy the result in one click
  • Free, no sign-up, and it runs entirely in your browser

Who uses this tool

The percent error calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

  • Reporting experimental error in a lab write-up
  • Checking an instrument against a known standard
  • Comparing an estimate with the actual outcome
  • Assessing how close a forecast turned out to be

Frequently asked questions

Should percent error be positive?

Conventionally yes, because the usual question is magnitude. Keep the sign when the direction of the error is the point, such as an instrument that always reads high.

Which value goes in the denominator?

The accepted value. Dividing by the measurement instead inflates or deflates the error depending on which direction you were wrong in.

What if the accepted value is zero?

Percent error is undefined, since you cannot divide by zero. Report the absolute error instead, which the calculator gives you.

If the percent error calculator is not quite what you need, these other free tools solve closely related problems, and there are 25 more in math calculators.

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Round to a place or multiple, with every rule compared.

Math Calculators Use tool

Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.