Percent Error Calculator
Compare a measured value against the true (accepted) value: percent error, absolute error, and relative error with the formula shown.
Details
Percent error
5%
A gap of 0.5
This works out how far a measurement is from the correct value, as a percentage. Enter what you measured and the accepted value.
It also handles percent difference, for when you are comparing two measurements and neither one is the 'right' answer.
What percent error measures
Percent error tells you how far a measurement sits from the value it should have been, expressed as a percentage of the correct value.
It is a measure of accuracy: how close you got to the truth. That is different from precision, which is how closely your repeated attempts agree with each other. You can be precise and consistently wrong.
The key requirement is that you have an accepted value to compare against: a published constant, a known concentration, a textbook figure. Without one, percent error is not the right tool.
You divide by the accepted value, never by your own measurement. Dividing by what you measured is the single most common mistake in lab reports.
What to enter
- What do you want to find?
- Percent error when you have a known correct value, or percent difference when comparing two measurements with no 'right' answer.
- Measured value
- What your experiment actually produced.
- Accepted value
- The published or theoretical value. This is what you divide by.
Percent error, difference and change
- Percent error
- One value is correct. Divide by the accepted value. Used in labs and instrument calibration.
- Percent difference
- Neither value is 'correct'. Divide by the average of the two. Used when comparing two methods or two trials.
- Percent change
- Something moved over time. Divide by the starting value. Used for prices and growth. See the percentage change calculator.
- Absolute vs relative
- Being 1 cm out matters enormously on a 10 cm object and not at all on a kilometre. Percent error is the relative version, which is why it is the one reported.
What this assumes
Percent error is usually reported as a positive number, since the size of the gap is what matters, not its direction.
The accepted value is genuinely accepted, not another measurement. If it is another measurement, use percent difference.
How to calculate percent error
Find the gap, divide by the accepted value, turn it into a percentage.
- | |
- Absolute value: ignore the minus sign
- ÷ accepted
- Always the accepted value, never your measurement
Subtract. Measured minus accepted. The sign tells you whether you overshot or undershot.
Take the absolute value. Drop the minus sign, since the size of the error is what is being reported.
Divide by the accepted value. This is the step to get right. Dividing by your own measurement gives a subtly wrong answer.
Multiply by 100. That converts the decimal into a percentage.
See a worked example: measuring gravity as 9.8 m/s²
- Measured
- 9.8 m/s²
- Accepted
- 9.81 m/s²
Gap: 9.8 − 9.81 = −0.01, so 0.01 as an absolute value.
Divide by the accepted value: 0.01 ÷ 9.81 = 0.00102.
Multiply by 100: 0.102%.
That is a very accurate result. Anything under 1% in a school lab is normally considered good.
0.102% error
Frequently asked questions
- What counts as a good percent error?
It depends entirely on the experiment. In a school lab, under 5% is usually considered good and under 1% is excellent.
In precision work the tolerances are far tighter, and in messy biological or field measurements 10% or more can be perfectly acceptable. Your lab instructions are the real answer.
- What's the difference between percent error and percent difference?
Percent error compares a measurement to a known correct value and divides by that accepted value.
Percent difference compares two measurements where neither is 'correct', and divides by their average. Comparing 48 and 52 gives an 8.0% difference, because you divide by 50.
If you find yourself unsure which to use, ask whether one of the two numbers is genuinely more trustworthy. If yes, it is percent error.
- Can percent error be negative?
It is normally reported as positive, using absolute value, because the size of the error is what matters.
Some courses do ask for a signed version, where negative means you measured low and positive means high. That extra information is useful for spotting systematic bias, so check what your instructor wants.
- Can percent error be over 100%?
Yes. It means your measurement was off by more than the accepted value itself, so measuring 25 when the answer is 10 gives 150% error.
A figure that large usually points to a unit mistake or a decimal in the wrong place rather than genuine measurement noise. Check the units first.
- What causes percent error in an experiment?
Systematic errors push every reading the same way: an uncalibrated scale, a consistently misread meniscus, air resistance you did not account for. These shift accuracy without hurting precision.
Random errors scatter readings either side: reaction time on a stopwatch, small fluctuations in conditions. Repeating and averaging reduces random error but does nothing for systematic error.
Problems people actually run into
Dividing by the measured value instead of the accepted one
The formula divides by the accepted value, always. Dividing by your own measurement produces a number that looks plausible and is wrong, which is why it survives to the final write-up so often.
With 9.8 against 9.81 the difference is tiny, but on a badly off measurement it is substantial: measuring 5 when the answer is 10 is 50% error, not 100%.
Reporting percent error when there is no accepted value
Comparing two of your own trials and calling it percent error is a category mistake. Neither trial is the truth, so there is nothing to be in error against.
That situation calls for percent difference, dividing by the average of the two. Using the wrong one in a report is an easy mark to lose.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026
