Percent Error Calculator
Technical Calculation Methodology and Tool
Percent Error Calculator
Compare a measured value against a known theoretical value
Every measurement contains some degree of inaccuracy. Percent error tells you exactly how far off your result is from the known correct value — and whether you overestimated or underestimated. Used by students, lab technicians, engineers, and quality control professionals worldwide.
What Is Percent Error?
Percent error compares an experimental or measured value to a true or accepted value. It expresses the discrepancy as a percentage of the true value, making it easy to assess accuracy regardless of scale.
The Percent Error Formula
The vertical bars indicate absolute value. By default, percent error is always positive. Use Directional mode in this calculator to see whether you overestimated or underestimated.
How to Use This Calculator
- Enter your experimental (measured) value in the first field.
- Enter the true or accepted value in the second field.
- Choose Absolute mode for a positive result, or Directional mode to see the direction.
- The percent error and full calculation breakdown appear instantly.
Worked Examples
You measure gravity as 9.65 m/s². The accepted value is 9.81 m/s².
Percent error: 1.63% (underestimate).
You calculate a molar mass as 58.2 g/mol. The actual is 58.44 g/mol.
Percent error: 0.41% — an excellent result.
A product is labelled 500g. Your scale reads 487g.
Percent error: 2.6% (underestimate).
What Is an Acceptable Percent Error?
This depends on the application. In pharmaceutical manufacturing, errors above 0.5% may be unacceptable. In a school chemistry lab, under 5% is generally good. In rough field measurements, 10% or more may be perfectly acceptable. Know your tolerance.
Overestimate vs Underestimate
When your experimental value is higher than the true value, you overestimated. When lower, you underestimated. The directional formula preserves this direction — positive means overestimate, negative means underestimate.
Common Mistakes to Avoid
- Using the wrong base: Always divide by the true value, not the experimental value.
- Confusing percent error with percent difference: Only use percent error when you have a known correct value to compare against.