Percentage error = (|estimate − true|/true) × 100%.
Key formulas
Keep the absolute value
Error size is positive whether you overshoot or undershoot.
Example
True 50 cm, measured 48 cm → 4% error.
Derived quantities
If length has 2% error and area uses length², relative area error is about 4% for small errors — mention this only when asked.
Worked example
Worked example
Problem. A true length is 50 cm; a measured length is 48 cm. Find the percentage error.
Solution. % error = (|48 − 50|/50) × 100% = 4%.
Zero true value
Percentage error is undefined if the true value is zero.
Review quiz
True 2.5 m, measured 2.6 m. Percentage error is:
Practice questions
- True 2.5 m, measured 2.6 m. Find % error.
- Why divide by the true value, not the measurement?
Study notes
Is absolute error enough?
Absolute error is |approx−true|; percentage puts it in context.