Percent error
01 // What it measures
Percent error compares a value you measured in a lab (the experimental value) to a known, accepted value (from a textbook, a reference table, or a more precise instrument). It tells you how far off your measurement was, as a percentage of the true value, so results with different units or sizes can still be compared fairly.
Percent error is always reported as a positive number — the formula uses absolute value, because being 5% too high and 5% too low are both a 5% error. If you need the direction of the error too, report it separately ("5% high").
02 // Worked example
A student measures the density of aluminum in a lab and gets 2.55 g/cm³. The accepted value is 2.70 g/cm³.
| Experimental value | 2.55 g/cm³ |
| Accepted value | 2.70 g/cm³ |
Formula percent error = |experimental − accepted| ÷ accepted × 100
03 // What counts as a good result
- In most middle and high school labs, under 5% error is considered good, and under 10% is usually acceptable for a hands-on measurement.
- Percent error is different from percent difference — percent difference compares two experimental values with no known "correct" answer, and divides by their average instead of an accepted value.
- A high percent error is not automatically a mistake. It is a signal to check your method: was a controlled variable off, was the tool read wrong, or was there a real limitation in the equipment?
04 // Questions people ask
Why is percent error always positive?
Because the formula takes the absolute value of the difference between your measurement and the accepted value, so a result that is too high and one that is too low by the same amount both give the same percent error.
What is a good percent error for a school lab?
There is no universal cutoff, but under 5% is usually considered a strong result, and under 10% is typically acceptable given the precision of school-grade equipment. Always check your teacher's rubric for the specific lab.
What is the difference between percent error and percent difference?
Percent error compares your result to a known, accepted value. Percent difference compares two experimental results to each other when there is no accepted value, dividing by the average of the two instead.