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Percent Error Calculator — Experimental vs Theoretical

Compare a measured (experimental) value against a known or expected (theoretical) value to find the percent error, commonly used in science and lab reports.

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Enter both values to calculate percent error
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Disclaimer: This tool performs standard mathematical calculations for educational and reference purposes. Always double-check results independently for graded coursework, exams, or safety-critical applications.

What Percent Error Tells You

Percent error measures how far a measured or experimental value is from a known, accepted, or theoretical value, expressed as a percentage. It's a standard way to report the accuracy of a measurement or experiment in physics, chemistry, and engineering lab work.

The Formula

% Error = |Experimental − Theoretical| / |Theoretical| × 100. Using the absolute value means percent error is normally reported as a positive number regardless of whether your measurement was higher or lower than expected — this calculator also shows the signed version so you can see the direction of the error.

Worked Example

A lab experiment measures the density of a substance as 2.68 g/mL, while the accepted theoretical value is 2.70 g/mL.

  1. Step 1: Difference = 2.68 − 2.70 = −0.02
  2. Step 2: |−0.02| / |2.70| = 0.0074
  3. Step 3: 0.0074 × 100 = 0.74% error

Frequently Asked Questions

It depends entirely on the field and equipment precision — under 5% is often considered acceptable for a basic school lab experiment, while precision engineering or analytical chemistry may require well under 1%. There's no universal threshold; compare against what's typical for your specific experiment type.
Percent error compares a measured value against a known, accepted theoretical value (like a textbook constant). Percent difference compares two experimental measurements against each other when neither is known to be the "true" value — it typically divides by the average of the two instead of a single reference value.
The standard formula uses absolute value, so percent error itself is reported as zero or positive. This calculator additionally shows the signed error so you can see whether your experimental value came in above or below the theoretical value.
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