Percent error = |measured − true| ÷ true × 100
In a lab you measure 9.8 m/s² for gravity, whose accepted value is 9.81. The difference is 0.01; divided by 9.81 that is 0.001, or 0.1%. The vertical bars mean absolute value: percent error is normally reported as a positive number, though some teachers keep the sign to show whether you measured high or low.
Percent error formula
|experimental − theoretical| ÷ theoretical × 100. Always divide by the accepted (true, theoretical) value, not the measured one.
Worked example
You measure the boiling point of water as 97.5 °C. Accepted: 100 °C. |97.5 − 100| = 2.5; 2.5 ÷ 100 × 100 = 2.5% error.
Percent error vs percent difference
Percent error compares a measurement to a known true value. Percent difference compares two measurements when neither is "correct", dividing by their average instead.
What is a good percent error?
It depends on the experiment. Under 5% is typical for school labs; precision instruments aim for under 1%.