Percent Error Calculator

The formula takes the absolute value of the difference between experimental and theoretical values before converting to a percentage, so it deliberately discards the direction of the error and reports only its magnitude. If you need to know whether your measurement ran high or low, look at the raw signed difference (experimental minus theoretical) separately from the percent error figure.

Dividing by the theoretical value anchors the percentage to the accepted standard you're comparing against, which keeps the metric consistent no matter how far off any individual measurement ran — every trial in a series gets judged against the same fixed reference point. Dividing by the experimental value instead would make the percentage shift depending on how wrong that particular measurement was, which defeats the purpose of a standardized accuracy metric.

It depends entirely on the experiment and equipment involved — a well-controlled physics experiment with precise instruments might target under 1-2% error, while a rougher classroom chemistry titration might reasonably see 5-10% and still be considered acceptable. There's no universal threshold; check your specific lab's guidelines or your instructor's expectations for what counts as acceptable accuracy for that particular measurement.

Percent error assumes one of your two values is the known, accepted true value and the other is your experimental attempt to match it, so the true value is what you divide by. Percent difference is used when neither of two measured values has a privileged 'true' status — comparing two independent experimental readings, for example — and typically divides by the average of the two instead of treating one as the standard.