Percent error shows how far a measurement is from the true value. Subtract the true value from the measured value, take the absolute value, divide by the true value and multiply by 100. Measuring 9.8 when the true value is 9.81 gives a percent error of about 0.10%.
Formula
% error = |measured − true| ÷ |true| × 100 Examples
- A scale reads 205 g for a 200 g weight: 5 ÷ 200 × 100 = 2.5% error.
- An estimate of 1,100 for an actual 1,250: 150 ÷ 1,250 × 100 = 12% error.
Common mistakes
- Dividing by the measured value instead of the true value.
- Keeping the sign when only the size of the error matters. The signed error is shown separately.
In Excel and Google Sheets
- Measured in A2, true value in B2 (format as %)
=ABS(A2-B2)/ABS(B2)
FAQ
Can percent error be negative?
The usual percent error is always positive. The signed error is negative when the measurement is below the true value.
What is a good percent error?
It depends on the field. In school labs, under 5% is often fine; precise instruments aim far lower.
Percent error or percentage difference?
Use percent error when one value is the accepted true value. Use percentage difference when both are measurements.
How we calculate: exact decimal math (no rounding errors), you choose the decimal places, and your numbers never leave your browser. Methodology →
Updated October 2026