Percent Error Calculator

Percent error measures how far an experimental or observed value deviates from the true or accepted value, expressed as a percentage of that reference value. It is a core metric in laboratory science, engineering, quality assurance, and forecasting — any field where you need to quantify how accurate a measurement or estimate is. A smaller percent error means greater accuracy. The result is always expressed as a positive number using absolute values, since direction matters less than magnitude in most accuracy assessments. Percent error is distinct from percent difference: percent error always has a known true or expected value as the reference, while percent difference compares two measured values symmetrically using their average as the denominator. Students in physics, chemistry, and biology labs routinely calculate percent error to assess how closely their experimental results match the theoretical or established values, while engineers and quality control professionals use it to verify that manufactured components, sensors, and instruments are operating within acceptable tolerances.

The calculator computes percent error using the standard formula: |actual − expected| ÷ |expected| × 100. First, the absolute difference between the actual (observed or measured) value and the expected (theoretical or true) value is calculated. Dividing by the absolute value of the expected value normalises the error relative to the true reference, and multiplying by 100 converts it to a percentage. Wrapping both the numerator and denominator in absolute values ensures the result is always a positive number, regardless of whether the actual value was higher or lower than expected. This is the widely accepted convention in science and engineering, where the magnitude of the deviation matters more than its direction. If directional information is important, the signed version of the formula can be used by removing the absolute values.