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Percent Error Calculator
Percent error, absolute error and relative error together.
Every measure of the gap
| Measure | Formula | Result |
|---|
Percent error or percent difference?
Use percent error when one value is authoritative — a known constant, a textbook figure, a calibrated reference — and you are measuring how far your result strayed from it. It divides by the true value.
Use percent difference when neither value is more correct than the other, such as two independent measurements of the same thing. It divides by the average of the two, because there is no reason to privilege either as the denominator. Choosing the wrong one is among the most common mistakes in undergraduate lab work.
Rough guide to accuracy
| Percent error | Typically considered |
|---|---|
| Under 1% | Excellent for a school or undergraduate lab |
| 1% – 5% | Good — the usual target |
| 5% – 10% | Acceptable, worth explaining in the write-up |
| Over 10% | Suggests a systematic error worth investigating |
Percent error
—
Other measures
Absolute error—
Relative error—
Signed percent error—
Percent difference—
—
Direction and rough quality of the result
Percent error is |experimental − true| ÷ |true| × 100. What counts as a good result depends entirely on the experiment: under 5 percent is usually fine in a school lab, while precision physics may need a small fraction of a percent.
Quick Tool Overview
The Percent Error Calculator compares a measured value against an accepted one and reports percent error, absolute error and relative error together. Percent difference is calculated too, for the case where neither value is the authoritative reference.
- What it does
- Calculates percent, absolute and relative error
- You give it
- An experimental value and a true or reference value
- You get back
- Percent error plus absolute, relative and percent difference
- Best for
- Lab reports, coursework and measurement checks
About the Percent Error Calculator
Compare a measured value against the accepted one and get percent error, absolute error and relative error at the same time. Percent difference is calculated too, for the case where neither of two values is the reference.
The distinction matters more than it first appears. Percent error assumes one value is correct and measures how far the other strayed from it, so it belongs in a lab report comparing your result to a known constant. Percent difference makes no such assumption and divides by the average of the two, which is the right choice when comparing two independent measurements. Using the wrong one is one of the most common mistakes in undergraduate lab work.
Key features of the Percent Error Calculator
Four error measures at once
Percent error, absolute error, relative error and percent difference, so you can use whichever the assignment asks for.
Percent difference done properly
When neither value is authoritative, dividing by the average is correct — and the tool explains when to use it.
Signed as well as absolute
A signed version shows whether the measurement ran high or low, which some courses require.
The formula shown
Each result prints its formula with your values substituted, ready to copy into a write-up.
Handles a zero true value
Percent error is undefined when the reference is zero, and the tool says so rather than showing infinity.
How to use the Percent Error Calculator
-
1
Enter the experimental value
What you actually measured or observed.
-
2
Enter the true value
The accepted, theoretical or reference value you are comparing against.
-
3
Read the errors
Percent error, absolute error and relative error, with the formula shown.
-
4
Use percent difference instead if needed
When neither value is authoritative, the percent difference figure is the appropriate one.
Frequently asked questions
Percent error is the absolute difference between the experimental and true values, divided by the true value, multiplied by 100. Written out: |experimental − true| ÷ |true| × 100.
Percent error compares a measurement to a known correct value. Percent difference compares two measurements where neither is authoritative, and divides by their average instead.
By the standard formula, no, because the numerator is an absolute value. Some courses use a signed version to indicate whether the measurement was high or low, which this tool also shows.
It depends entirely on the experiment. In a school lab under 5 percent is usually considered good, while precision physics may require a small fraction of a percent.
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