This percent error calculator measures how far a measured or experimental value is from the true value, as a percentage. Enter both values and get the percent error, the absolute error, and the step-by-step working.
What Is Percent Error?
Percent error tells you how inaccurate a measurement is, relative to the true value, expressed as a percentage. It is the standard accuracy measure in science labs: you compare what you measured against what you should have gotten. A percent error under 1% is excellent for school experiments; under 5% is usually acceptable; large errors point to a method or instrument problem. It is part of our Math Calculators collection.
The Percent Error Formula
Percent Error = |Measured − True| ÷ |True| × 100%
Worked example (verified): you measure a solution’s concentration as 24.5 g/L; the true value is 25 g/L. |24.5 − 25| = 0.5 → 0.5 ÷ 25 = 0.02 → 2.00% error, or 98% accuracy. The absolute-value bars matter: percent error is always reported as a positive number, because it describes the size of the error, not its direction. (If you need the direction too, that signed version is just percent difference/change — see our Percentage Calculator.)
Percent Error vs. Percent Difference vs. Percent Change
- Percent error: one value is the accepted truth (lab measurements vs. a known standard).
- Percent difference: neither value is “true” — two measurements of the same thing are compared using their average as the denominator.
- Percent change: one value is the earlier/starting value — growth and decline over time.
Choosing the right denominator is the whole game; percent error always divides by the true value.
Why Measurements Have Error
- Instrument limits: a ruler marked in millimeters cannot honestly report hundredths.
- Reading error: parallax, reaction time, meniscus reading.
- Systematic error: an uncalibrated scale shifts every reading the same way — percent error exposes it.
- Random error: small unpredictable fluctuations; repeated trials and averages tame it, and spread is measured with the Standard Deviation Calculator.
Related Tools
- Percentage Calculator — everyday percentage problems
- Standard Deviation Calculator — spread of repeated measurements
- Scientific Notation Calculator — tiny and huge lab numbers
Frequently Asked Questions
Can percent error be negative?
The standard percent error formula uses absolute value, so it is always positive. Some classes compute a signed version (measured − true) to show direction; check which your teacher expects.
What is a good percent error?
In school chemistry and physics, under 5% is generally good and under 1% excellent. Professional instruments target far lower, depending on the field.
What if the true value is zero?
Percent error is undefined — you cannot divide by zero. Report the absolute error instead.
What is the difference between absolute and relative error?
Absolute error is the raw gap (0.5 g/L). Relative error is that gap divided by the true value (0.02); percent error is the relative error × 100.
How do I calculate percent error in Excel?
=ABS(measured − true)/ABS(true), formatted as a percentage.
Is this percent error calculator free?
Yes — free, instant, no sign-up.