Percent Error Calculator
Calculate percent error between a measured or estimated value and the true value — with absolute error, signed relative error, and step-by-step formula.
Frequently Asked Questions
How do you calculate percent error?
Percent error = |measured − actual| ÷ |actual| × 100. Take the absolute difference between your measured (or estimated) value and the true value, divide by the true value, and multiply by 100. Example: measuring 9.5 cm when the true length is 10 cm gives |9.5 − 10| ÷ 10 × 100 = 5% error.
What is the difference between percent error and percent difference?
Percent error compares a measurement against a known true or accepted value — it measures accuracy. Percent difference compares two equally valid measurements against their average — it measures agreement when neither value is the reference. Use percent error in labs and quality control where a standard exists; percent difference when comparing two experimental results.
Can percent error be negative?
The standard percent error uses absolute values, so it is always positive. However, the signed (relative) error keeps the direction: positive means an overestimate, negative means an underestimate. This calculator shows both — the signed error is useful for spotting systematic bias, such as an instrument that consistently reads high.
What is an acceptable percent error?
It depends entirely on context. In high-school chemistry labs, under 5–10% is generally considered good. Engineering tolerances are often under 1%, and analytical chemistry or metrology may require under 0.1%. In business forecasting, errors of 10–20% can be normal. Always compare your error against the standard for your field.