Percent Error Calculator
Compare a measured value with the accepted one and get the percent error, the absolute error and the signed error for lab reports.
How to use it
- Enter the accepted or theoretical value first, then what you actually measured.
- Percent error is always reported as a positive number; the signed error tells you the direction.
- A negative signed error means you measured low.
Examples
| Exact 50, measured 47.5 | 5% error |
| Exact 9.81, measured 9.75 | 0.61% error |
Percent error tells you how far a measurement strayed from the accepted or theoretical value, scaled so a lab report can compare results across completely different quantities — grams, seconds, metres per second squared — on the same footing.
It always comes back positive; the signed error alongside it tells you whether the measurement ran high or low, which matters when writing up a source of systematic error.
What counts as acceptable, by field
| Context | Typical tolerance |
|---|---|
| School physics lab (e.g. measuring g) | Under 5% |
| Undergraduate chemistry titration | Under 1–2% |
| Manufacturing / precision engineering | Under 0.1% |
| Calibration-grade instrumentation | Under 0.01% |
Worked example
Measuring free-fall acceleration and getting 9.75 m/s² against the accepted 9.81 m/s²: percent error = |9.75 − 9.81| ÷ 9.81 × 100 = 0.61%. The signed error is −0.61%, meaning the measurement ran low — worth mentioning air resistance or timing lag as a likely cause in the write-up.
Questions people ask
What counts as an acceptable percent error?
It depends entirely on the experiment. School lab work often accepts under 5%; precision engineering may need under 0.1%. Compare against the tolerance you were given rather than a universal rule.
Is anything sent to a server?
No. Every calculation runs in your browser, so nothing you type is uploaded and the page keeps working offline once it has loaded.
Last updated 17 August 2026