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Percent Error Calculator

Compare an observed or measured value with the true value to see the absolute error and the percent error. It is widely used in science labs and quality checks.

Formulas last reviewed: September 2026

Disclaimer: Results are calculated with standard formulas and are meant for learning and quick checks. Please double-check important work, especially anything used for exams, engineering, finance or safety.

Measuring how close you got

Percent error tells you how far a measured or experimental value is from the true, accepted value, as a percentage of the true value. It is used in school science labs, engineering checks and quality control.

A small percent error means your measurement is accurate. This calculator also shows the absolute error and whether your value was above or below the true one.

How it is calculated

Percent error = |observed − true| ÷ |true| × 100

Worked example

Measuring gravity as 9.8 m/s² when the accepted value is 9.81

Absolute error
0.01
Percent error
0.1%

The measurement is about 0.1% below the accepted value.

Percent error vs percent difference

Use percent error when one value is the accepted or true value. Use percent difference when you are comparing two measurements and neither is more correct than the other; that one divides by the average of the two.

  • Absolute error is the size of the gap in the original units.
  • Signed error keeps the direction, so a negative value means you measured too low.

Frequently asked questions

Can percent error be negative?

The usual formula uses the absolute value, so percent error is never negative. The signed error shown separately tells you whether you were above or below the true value.

What is a good percent error?

It depends on the field. In school labs under 5% is usually considered good, while precision engineering may demand well under 1%.

What if the true value is zero?

Percent error is not defined when the true value is zero because you would divide by zero. Use the absolute error instead.

How is it different from percent change?

Percent change compares a new value with an old one, while percent error compares a measurement with a known correct value.