Percent Error Calculator
Compare observed measurements to known values and calculate percent error, percent difference, and accuracy.
Measurement comparison
Absolute error:
Percent error:
Percent difference:
Within tolerance:
Percent difference uses the average of the observed and expected absolute values, giving a symmetric measure of deviation.
Percent error vs. percent difference
Percent error compares a measured value against a known, "true" or accepted reference value — it's used when you know the correct answer and want to see how far off a measurement or experiment result was. Percent difference is used instead when comparing two measured values where neither is definitively "correct," so it divides the absolute difference by the average magnitude of both values rather than by a single reference value.
Frequently asked questions
When would percent error be undefined? If the expected (true) value is exactly zero, dividing by it is mathematically undefined — in that case, use percent difference instead, or report the absolute error directly.
What's considered a "good" percent error in a lab experiment? It depends heavily on the field and equipment — some physics experiments target well under 1%, while other measurements (biological samples, field data) may reasonably have several percent error and still be considered acceptable.
What does the "tolerance" field do? It lets you set an acceptable percent error threshold for your specific application (like a manufacturing spec or lab requirement) — the calculator then tells you directly whether your result falls within that tolerance.