Percent Error Calculator

What is a "good" percent error?

It depends on the field. In chemistry lab work, errors under 5% are generally acceptable. In physics, 1–2% is often the standard. In industrial manufacturing, tolerances can be as tight as 0.01%. The acceptable error depends on the precision of your instruments and the requirements of the application.

Does the sign of percent error matter?

Yes. A positive percent error means your experimental value is higher than the accepted value (e.g., you measured more mass than expected). A negative error means it is lower. The sign helps identify systematic errors — if your results are always high, your scale may be miscalibrated.

What is the difference between accuracy and precision?

Accuracy is how close a measurement is to the true value — measured by percent error. Precision is how repeatable/consistent measurements are — measured by standard deviation. A result can be precise (low scatter) but inaccurate (far from truth), or accurate on average but imprecise (high scatter).

Can I use this for financial forecasting accuracy?

Yes, though the metric is then called Mean Absolute Percentage Error (MAPE) when averaged across many forecasts. For a single forecast: % error = (Actual − Forecast)/|Actual| × 100. Note: the denominator is the actual (theoretical) value, not the forecast — identical to the scientific percent error formula.