Percent Error Calculator
Technical Calculation Methodology and Tool
Percent Error Calculator
Compare a measured value against a known theoretical value
Interactive percent error calculator visualizer
Enter your theoretical and experimental values using the sliders and watch the bullseye visualizer instantly show how far your result is from the target. The dot moves outward from the center as percent error increases — a clear, intuitive way to understand experimental accuracy.
Percent error
6.00%
Difference
6
Accuracy
94.00%
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📹 Video Walkthrough — Percent Error Calculator
Every measurement deviates from the true value — the question is by how much. Use this Percent Error Calculator to calculate how far your experimental result is from the accepted value using just 2 inputs: your measured value and the known correct value. It matters across physics labs, chemistry experiments, quality control in manufacturing, and engineering validation. This page includes the formula, worked examples, plain-English theory, and an FAQ.
What is percent error?
Percent error is a number that tells you how close a measurement is to the correct value. It expresses the gap between your result and the true value as a percentage — so you can instantly see how accurate your measurement was, regardless of the units or scale involved.
Simple Explanation
Think of it like hitting a target. The true value is the bullseye. Your measurement is where your arrow actually landed. Percent error measures how far off-centre you were — not in raw units, but as a percentage of the distance to the target. A small percent error means you were close; a large one means you missed by a lot.
The Percent Error Formula
Use the formula below to calculate percent error.
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 of the error.
- Click Calculate to see your result.
Simple Example
You measure a resistor and get 98 Ω. The true value is 100 Ω.
% Error = (|98 − 100| ÷ 100) × 100 = 2%
Your measurement was 2% below the true value — a small underestimate.
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.