Compare your experimental measurement to the accepted theoretical value to quantify how accurate your experimental technique was.

Calculate percent error for lab reports and homework problems that require evaluating the accuracy of experimental results.

Compare measured product dimensions to the design specification to determine whether parts are within acceptable tolerance.

Verify that a calculated or simulated result matches an expected theoretical or benchmark value to within acceptable error bounds.

Assess the accuracy of sensor readings, instrument calibrations, or model predictions by comparing against a known reference value.

Always identify your theoretical value correctly before calculating. The theoretical value is the accepted, standard, or known-correct value — not simply the larger number. If you are measuring the density of water, the theoretical value is 1.00 g/mL (at 4°C), regardless of what you measured experimentally.

Percent error does not tell you the direction of error — only the magnitude. If knowing whether your measurement was systematically too high or too low is important (it usually is), calculate signed percent error: (experimental − theoretical) / theoretical × 100% (without absolute value). Positive = overestimate; negative = underestimate.

When multiple measurements lead to high percent error, investigate systematic error (bias) vs. random error (noise). Systematic errors affect all measurements in the same direction (e.g., an uncalibrated balance). Random errors scatter around the true value. Systematic errors cannot be averaged away — they require identifying and fixing the source.

In analytical chemistry and metrology, "uncertainty" is preferred over "error" because it is more precisely defined. Measurement uncertainty includes both Type A (statistical) and Type B (systematic) components. Percent error is a simplified approximation used in educational settings; professional labs report expanded uncertainty at a specified confidence level.