How to Calculate Percent Error
By ToolNimba Editorial Team June 20, 2026 2 min read
Percent error tells you how far a measured or experimental value is from the true or accepted value, expressed as a percentage. It is one of the most common calculations in science labs, because it turns a raw gap into a number you can compare across experiments.
The formula
percent error = ( absolute value of (experimental minus theoretical) divided by theoretical ) times 100.
Step by step
- Subtract the theoretical (true) value from your experimental value.
- Take the absolute value, so the result is positive.
- Divide by the theoretical value.
- Multiply by 100 to get a percentage.
A worked example
Suppose the accepted density of a substance is 8.00 and you measured 7.84.
- Difference: 7.84 minus 8.00 equals minus 0.16.
- Absolute value: 0.16.
- Divide by the true value: 0.16 divided by 8.00 equals 0.02.
- Multiply by 100: 2 percent error.
What counts as a good result?
It depends on the field, but in a school science lab a percent error under about 5 percent is usually considered good, and under 10 percent is acceptable. A large percent error points to a measurement problem, a flawed method, or a mistake in the accepted value you are comparing against.
Always use the absolute value
Percent error is reported as a positive number. The absolute value step makes sure an under-measurement and an over-measurement of the same size give the same error.
Calculate it instantly
Enter your experimental and true values below and the calculator returns the percent error with the steps.
๐ฏ Try the free tool Percent Error Calculator Free percent error calculator: enter your measured and actual values to get the percent error and absolute error instantly, with formula and worked examples.Frequently asked questions
What is the percent error formula?
Percent error equals the absolute value of (experimental minus theoretical), divided by the theoretical value, times 100.
Can percent error be negative?
No. Percent error uses the absolute value of the difference, so it is always reported as a positive number. If you skip the absolute value you get relative error, which can be negative.
What is a good percent error?
In a typical science lab, under 5 percent is considered good and under 10 percent is acceptable. Acceptable ranges vary by field and the precision of the equipment.