Percent Error Calculator

Compare an experimental or measured value with the accepted true value. Get the percent error, the absolute error and the signed error in one step.

Percent error
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Absolute error–
Relative error–
Signed error–

The percent error formula

Percent error = |measured − true| ÷ |true| × 100

The top part, |measured − true|, is the absolute error: how far off you were in the original units. Dividing by the true value gives the relative error, and multiplying by 100 turns it into a percentage.

Worked example: measuring gravity

In a pendulum lab, a student measures the acceleration due to gravity as 9.6 m/s². The accepted value is 9.81 m/s².

  • Absolute error: |9.6 − 9.81| = 0.21 m/s²
  • Relative error: 0.21 ÷ 9.81 ≈ 0.0214
  • Percent error: 0.0214 × 100 ≈ 2.14%

The measurement was a little low, so the signed error is −2.14%.

How to use it

  1. Enter the value you measured or estimated.
  2. Enter the accepted or theoretical value, often from a textbook or data table.
  3. Read the percent error. The signed error shows whether you were above or below.

Common sources of error in labs

  • Measurement limits: rulers, scales and timers can only be read so precisely.
  • Reaction time: starting and stopping a stopwatch by hand adds a small delay.
  • Conditions: temperature, air resistance or friction can differ from the ideal assumptions.
  • Calculation slips: rounding too early can add error. Keep extra digits until the final step.

Comparing two measurements without a known correct value? Use percent difference on the percent change calculator.

Last reviewed October 2026.

Frequently asked questions

What is percent error?

Percent error measures how far a measured or estimated value is from the true or accepted value, as a percentage of the true value. It tells you how accurate an experiment or estimate was.

Can percent error be negative?

The standard formula uses the absolute value, so percent error is always zero or positive. Some teachers keep the sign to show whether the measurement was too high (positive) or too low (negative). The calculator shows both.

What is a good percent error?

It depends on the task. In many school labs, an error under about 5% to 10% is considered good, while precise scientific or engineering work may need errors well under 1%. Check what your teacher or field expects.

What is the difference between percent error and percent difference?

Percent error compares a measurement with a known correct value. Percent difference compares two measurements when neither is known to be correct, and divides by their average instead.

Why can't the true value be zero?

Percent error divides by the true value, and dividing by zero is undefined. If the accepted value is zero, report the absolute error (the plain difference) instead.