Calculate the percent error between an experimental and true value.
Calculated locally in your browser.
How do you calculate percent error?
Percent error = |experimental − theoretical| ÷ |theoretical| × 100. It compares an experimental measurement to the true value, where a smaller percent error means a more accurate result. For example, an experimental value of 98 against a true value of 100 is a 2% error, alongside the absolute error.
Understanding your result
A smaller percent error means a more accurate measurement.
Formula and method
Percent error = |experimental − theoretical| ÷ |theoretical| × 100.
Assumptions and limitations
It compares one experimental value against one accepted or theoretical value and reports the size of the difference as a percentage. It says nothing about why the error occurred, and it cannot be used when the true value is zero, since dividing by zero is undefined.
Worked example
An experimental value of 98 against a true value of 100 is a 2% error.
How to use this tool
- Enter the experimental value.
- Enter the true (theoretical) value.
Common mistakes to avoid
- Using zero as the theoretical value (undefined).
About the Percent Error Calculator
Measure how far an experimental result is from the true value as a percentage.
Who should use this tool
Science and chemistry students reporting how close a measured result is to the accepted value, lab teachers marking practical work, and anyone comparing an experimental reading against a known standard to express the gap as a clear percentage.
Benefits
- Expresses measurement accuracy as a single clear percentage
- Uses the absolute difference so the result is never misleadingly signed
- Makes comparing results across experiments straightforward
- Runs free in the browser with no account needed
Practical use cases
- Reporting accuracy in a chemistry or physics lab write-up
- Comparing a measured constant against its accepted value
- Grading how close students' experimental results are
- Checking a calibration reading against a reference standard
Frequently asked questions
Is percent error always positive?
The standard definition uses absolute values, so it is reported as a non-negative percentage.
Why is percent error shown as a positive number?
Because the formula takes the absolute value of the difference before dividing, so direction is ignored. Whether your measurement was above or below the true value, the result describes how far off it was, not which way. This keeps the focus on accuracy alone.
What is a good percent error?
A smaller percent error means a more accurate measurement, and what counts as acceptable depends entirely on the experiment and its precision requirements. There is no single universal threshold; a careful lab may aim for a very low figure, while rough estimates tolerate more.