Use our Percent Error Calculator to quickly compare a measured (experimental) value to an accepted (true) value and express the difference as a percentage. Whether you are in a chemistry lab, calibrating instruments, or checking forecast accuracy, this tool gives clear, rounded results in seconds.
What is Percent Error?
Percent error quantifies how far a measured value is from the true or accepted value. It is a standardized way to report measurement accuracy, making it easy to compare results across different scales and units. A small percent error indicates that your measurement is close to the accepted value, while a larger percent error signals greater deviation.
The Percent Error Formula
The most common definition uses the absolute difference between the measured and true values, divided by the absolute true value, then multiplied by 100 to convert to a percentage:
- Absolute percent error = |measured ? true| ÷ |true| × 100%
Some contexts also look at the direction of the error (whether you overestimated or underestimated). In that case, you can use a signed version:
- Signed percent error = (measured ? true) ÷ true × 100%
Our Percent Error Calculator supports both approaches. By default, it calculates absolute percent error; you can opt in to the signed version if you need the direction.
How to Use the Percent Error Calculator
- Enter your measured (observed) value.
- Enter the accepted (true) value.
- Choose how many decimal places to display (0–10).
- Optionally check the box to show signed percent error.
- Click “Calculate Percent Error.”
Within a moment, you will see the difference, relative error, and the percent error based on your settings. If your true value is zero, the calculation is undefined; the calculator will let you know to avoid misleading results.
Worked Example
Suppose you measured the density of a liquid as 0.98 g/mL, and the accepted value is 1.00 g/mL.
- Difference: 0.98 ? 1.00 = ?0.02
- Absolute difference: 0.02
- Absolute percent error: 0.02 ÷ 1.00 × 100% = 2.00%
- Signed percent error: ?0.02 ÷ 1.00 × 100% = ?2.00%
If you select signed output, you will see ?2.00%, indicating the measured value is lower than the accepted value. Otherwise, you will see 2.00% to emphasize magnitude over direction.
Why Percent Error Matters
Percent error is a core metric for evaluating the quality of measurements and predictions across science, engineering, manufacturing, and analytics. It helps you:
- Diagnose instrument calibration issues
- Compare results across different units and scales
- Communicate accuracy clearly to stakeholders
- Prioritize improvements where deviations are largest
Because percent error is unitless, it allows you to fairly compare accuracy across varied measurements, from temperature to mass to reaction yield.
Common Mistakes to Avoid
- Dividing by the measured value instead of the true value
- Forgetting to use absolute values for the standard definition
- Reporting too many decimal places, which implies false precision
- Calculating percent error when the true value is zero (undefined)
Our Percent Error Calculator helps prevent these errors by providing a clear workflow, optional signed output, and a simple way to control rounding.
Tips for Better Results
- Repeat measurements to reduce random error and average the results.
- Calibrate instruments regularly and record environmental conditions.
- Pick a sensible number of decimal places—2 to 4 is plenty for most reports.
- State your accepted value source so others can verify your comparison.
With these best practices and our easy Percent Error Calculator, you can present accurate, transparent results that are simple for anyone to interpret.