Quick answer: Measuring gravity as 9.81 m/s^2 against the accepted 9.80 m/s^2 gives a percent error of 0.102%, which is excellent lab accuracy. Enter your experimental and true values above to check your own measurement instantly.
Last reviewed: November 2026
Key facts
- Percent error = |experimental - true| / |true| x 100%.
- It measures accuracy (closeness to truth), not precision (repeatability).
- Under 1% is excellent, under 5% is good, under 10% is acceptable for most school labs.
- The true value can never be zero: division by zero is undefined.
A percent error calculator compares your experimental measurement against the accepted true value. Enter both values to get the absolute error, the percent error, and a plain-English interpretation of how accurate your result is.
Every lab report asks the same question: how close did you get? Percent error answers it as a single number that works across experiments, because it scales the mistake against the size of the thing being measured. Missing by 0.1 on a 10-unit measurement is a 1% error; missing by 0.1 on a 0.5-unit measurement is a 20% error, and the percent form makes that obvious. This free tool computes it from your two values and tells you whether the result counts as good.
How to use the percent error calculator
- Enter your experimental (measured) value: what your experiment produced.
- Enter the true (accepted) value: the textbook or reference value.
- Click Calculate to see the absolute error, percent error, and interpretation.
Percent error formula
The absolute value makes the error positive by convention: the sign of (experimental - true) tells you whether you overshot or undershot, but the reported percent error is the magnitude. Dividing by the true value normalizes the mistake, and multiplying by 100 turns it into a percent. Note the denominator is the true value, not your measurement: the error is always judged against the accepted standard.
Worked examples
Example 1: measuring gravity, 9.81 vs. 9.80 m/s^2
Absolute error = |9.81 - 9.80| = 0.01 m/s^2. Percent error = 0.01 / 9.80 x 100% = 0.102%. Interpretation: excellent, well within 1%.
Example 2: aluminum density, 2.42 vs. accepted 2.70 g/cm^3
Absolute error = |2.42 - 2.70| = 0.28 g/cm^3. Percent error = 0.28 / 2.70 x 100% = 10.37%. Interpretation: high, above the 10% school-lab bar, so the procedure deserves a review.
Percent error interpretation table
| Percent error | Rating | What it means |
|---|---|---|
| Under 1% | Excellent | Strong technique, publishable quality |
| 1% - 5% | Good | Solid school or undergraduate lab result |
| 5% - 10% | Acceptable | Usable, but note possible error sources |
| 10% - 20% | High | Review procedure and equipment |
| Over 20% | Very high | Likely a method or calculation error |
Tips for accurate percent error results
- Use the most authoritative true value you can find: a textbook constant beats a classmate's result.
- Keep units identical on both values: grams vs. kilograms silently inflates the error a thousandfold.
- Report the sign separately if your teacher wants it: a -2% error (undershoot) and +2% (overshoot) suggest different problems.
- Pair percent error with percent difference when comparing two of your own trials against each other instead of a known truth.
Common percent error mistakes to avoid
- Dividing by the experimental value: the denominator must be the true value. Dividing by your own measurement judges the error against the wrong standard.
- Forgetting the x 100: 0.05 is the relative error; 5% is the percent error. Lab reports expect the percent form.
- True value of zero: percent error is undefined when the accepted value is zero, since you cannot divide by zero.
- Confusing accuracy with precision: three identical-but-wrong readings are precise but inaccurate. Percent error catches accuracy only.
Frequently asked questions
What is the percent error formula?
Percent error = (|experimental value - true value| / |true value|) x 100%. Subtract, take the absolute value, divide by the true value, and multiply by 100.
What is a good percent error in a lab?
Under 5% is good for most school labs, and under 1% is excellent. Between 5% and 10% is acceptable but worth discussing error sources; above 10% usually signals a procedure or equipment problem.
Can percent error be negative?
By standard convention, no: the absolute value makes it positive. Some teachers ask for the signed version, where negative means your measurement came in below the true value and positive means above it.
What is the difference between absolute error and percent error?
Absolute error is the raw difference (|experimental - true|) in the original units. Percent error divides that by the true value and multiplies by 100, so it expresses the mistake relative to the size of what was measured.
What is the difference between accuracy and precision?
Accuracy is closeness to the true value, which percent error measures. Precision is how repeatable your measurements are. You can be precise without being accurate (consistent but wrong) or accurate without being precise (scattered around the truth).
What is the difference between percent error and percent difference?
Percent error divides by the known true value, so it needs an accepted standard. Percent difference divides by the average of the two values and is used when neither value is the standard. For an experimental 10.3 against a true 10, percent error is 0.3 / 10 x 100% = 3.00%, while percent difference is 0.3 / 10.15 x 100% = 2.96%.
What percent error is acceptable in a lab?
There is no universal standard: it depends on the field and the instruments. As a guide, under 1% is excellent, under 5% is good for school and undergraduate labs, physics labs often aim under 1-2%, and analytical chemistry under 0.5%. Above 10% usually means reviewing the procedure or equipment.
Related calculators
- Standard Deviation Calculator: measure how spread out repeated measurements are.
- P-Value Calculator: test whether a lab result is statistically significant.
- Z-Score Calculator: see how many standard deviations a measurement sits from the mean.
Sources
- OpenStax: Chemistry 2e textbook, measurements, accuracy, and precision chapter
- NIST: National Institute of Standards and Technology, measurement accuracy reference
Key takeaways
- Bookmark this tool for quick recalculations whenever your inputs change.
- Percent error = (|experimental - true| / |true|) x 100%.
- It measures accuracy against the accepted value, not precision.
- Under 1% excellent, under 5% good, over 10% needs a procedure review.
- Always divide by the true value, and never use a true value of zero.
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