Percent Error Calculator

It measures how close a measured or estimated value is to the true value, as a percentage. A small percent error means the measurement is accurate; a large one means it is far off.

Percent error = |measured − expected| ÷ |expected| × 100. You take the difference, divide by the true value, and multiply by 100 to express it as a percentage.

The expected (or true, accepted, theoretical) value is the reference you are comparing against — for example a textbook constant or a known standard. The measured (or observed, experimental) value is what you actually got.

Because the standard definition uses the absolute value of the difference — it reports the magnitude of the error, not its direction. This tool adds an "over" or "under" label so you still know which way it went.

Percent error compares a measurement against a known true value. Percent difference compares two measurements when neither is the "correct" one, dividing by their average instead of by a reference value.

Because it is the denominator. Dividing by zero is undefined, and a true value of zero has no meaningful percentage to be off by. Use absolute error in that case instead.

Yes. The tool uses the magnitude of the expected value in the denominator, so a reference like −40 still yields a sensible percent error. The signed difference is shown separately.

It depends on the field and the equipment. In a school lab, under 5% is often considered good; precision instruments aim for a fraction of a percent. There is no universal threshold.

Relative error is the same ratio without multiplying by 100 — a fraction rather than a percentage. Percent error is just relative error expressed as a percent.

Yes. The calculation runs in your browser; the values you enter are never uploaded.