How percent error is calculated
Percent error scales the gap between a measured value and a known reference so you can compare accuracy regardless of the units or magnitude involved. You take the absolute difference between the experimental and actual values, divide by the absolute actual value, then multiply by 100.
Because the difference is taken in absolute value, the result is always reported as a non-negative percentage. It answers the question "how far off was my measurement, relative to the truth?"
Reading the result
A smaller percent error means a more accurate measurement, with 0% indicating a perfect match. There is no universal cutoff for "good" — what counts as acceptable depends entirely on the context.
- Under 1%: typically excellent for most lab and field work.
- A few percent: often fine for rough estimates and casual measurements.
- Large values: signal a calibration problem, a mistake, or the wrong reference.
Practical tips
Always make sure the actual value really is your most trustworthy reference — a textbook constant, a certified standard, or an agreed-upon true value. Percent error is only as meaningful as the baseline you divide by.
- Repeat measurements and average them before computing the error.
- Keep enough significant figures so rounding does not inflate the result.
- Use percent error to compare methods, not to declare a single trial right or wrong.
Common mistakes
A frequent error is swapping the roles of the two values — dividing by the experimental value instead of the actual one — which changes the answer. The denominator should always be the known true value.
- Divide by the actual value, not the measured one.
- An actual value of zero makes the formula undefined and is rejected.
- Do not confuse percent error (accuracy) with percent difference between two unknowns.
Formula
percent error = |experimental − actual| / |actual| · 100