Percentage error calculator
Compare an experimental or estimated value with the accepted true value — the standard accuracy measure in science classes and labs.
Percent error
Example: measured 9.5, true value 10
How to calculate percent error
- Subtract the true value from the measured value and take the absolute value.
- Divide by the absolute value of the true value.
- Multiply by 100.
Example: measured 9.5 against a true value of 10 → 0.5 ÷ 10 × 100 = 5%
Examples
What counts as a good percent error?
It depends on the field. In a school chemistry lab, under 5% is usually considered good. Precision engineering may demand under 0.1%, while rough estimates in economics can tolerate 10% or more. Always compare your error to the tolerance your context requires.
Frequently asked questions
Can percent error be negative?
With the standard formula, no — the absolute value makes it positive. Some teachers use a signed version to show direction: negative means you measured low, positive high.
What is the difference between percent error and percentage difference?
Percent error compares against a known true value. Percentage difference compares two values of equal standing, dividing by their average.
What if the true value is zero?
Percent error is undefined — you cannot divide by zero. Use absolute error instead.
Is percent error the same as percent uncertainty?
No. Percent error compares your result to the accepted value; percent uncertainty expresses the resolution limits of your measuring instrument.
Why is my percent error over 100%?
Your measurement is more than double (or less than zero times) the true value — usually a sign of a unit conversion mistake or a systematic error in the experiment.