Percent Error Calculator
Calculate the percent error between a measured value and the accepted or theoretical value.
How this calculator works
Percent error expresses how far a measured or experimental value is from the accepted or theoretical value, as a percentage.
A smaller percent error indicates a more accurate measurement.
Percent error in scientific and practical contexts
Percent error is widely used in science labs to evaluate how close an experimental measurement came to a known or theoretical value, helping identify measurement issues or experimental flaws. Lower percent error generally indicates more precise or accurate measurement technique.
Outside formal science, the same calculation is useful anytime you're comparing an estimate or prediction against an actual outcome — like comparing a budget forecast to actual spending, or a project time estimate to actual completion time.
A worked example
A lab measures the boiling point of water as 99.1 degrees C, against a theoretical value of 100 degrees C: percent error equals the absolute value of 99.1 minus 100, divided by 100, times 100, or 0.9% — indicating a fairly precise measurement.
If a different measurement came in at 95 degrees C instead, percent error would jump to 5% — five times higher, illustrating how percent error scales directly with the size of the discrepancy from the expected value.
Percent error in scientific measurement
Percent error is a standard way to express measurement accuracy in science and engineering — a smaller percent error indicates a measurement closer to the accepted or theoretical value, while larger errors may indicate equipment issues, procedural mistakes, or genuine experimental limitations.
It's worth distinguishing percent error from percent difference: percent error compares a measured value against a known, accepted value, while percent difference compares two experimental measurements against each other where neither is known to be more 'correct.'
Frequently asked questions
Can percent error be negative?
This calculator shows the absolute (unsigned) percent error, which is the standard convention in most science and engineering contexts.
What's considered an acceptable percent error?
This depends entirely on the field and application — some scientific contexts expect well under 1% error, while other estimation contexts might consider 10-20% reasonable. There's no universal standard.
Can percent error be greater than 100%?
Yes — if the experimental value is more than double the theoretical value (or reflects a very large discrepancy), percent error can exceed 100%.
What's a 'good' percent error in an experiment?
It depends heavily on the field and equipment used — some physics experiments target under 1%, while more variable biological or environmental measurements might reasonably expect a higher acceptable range.