Percent Error Calculator

What It Does

Compare a measurement with an accepted (theoretical) value, or compare two measurements with each other. The tool returns the percentage, the absolute difference and the side the first value fell on, and shows the arithmetic with the reader's own figures.

Real-World Usage

Check a lab measurement against a published constant, grade the accuracy of a set of readings, compare two instruments measuring the same thing, or report how far a calculation missed a reference figure.

Local Context

Percent error and percent difference are the same arithmetic everywhere; only the decimal separator changes with the page's language. Which value counts as the accepted one is the reader's call — a textbook constant, a manufacturer's rating, or the mean of several runs — and the calculator takes it as an input rather than assuming one.

How do I calculate percent error?

Subtract the accepted value from the measured value, take the absolute value, divide by the accepted value and multiply by 100. A measurement of 4.9 against an accepted 5.0 is |4.9 − 5.0| ÷ 5.0 × 100 = 2 %.

What is the difference between percent error and percent difference?

Percent error needs one value to be the accepted, theoretical one, so it divides by that value. Percent difference compares two measurements where neither is accepted, so it divides by the mean of the two magnitudes — which makes it symmetric: swapping the two values gives the same answer.

Can percent error be negative?

The percentage is always reported as a magnitude, never negative. Whether the measurement was above or below the accepted value is reported separately, so a 2 % error below the accepted value reads 2 % with the low side named.

Why can't I use a zero accepted value?

Percent error divides by the accepted value, so an accepted value of zero has no error fraction — the result would be a division by zero. The calculator refuses that input rather than returning infinity or an invented number.