Your result will appear here
Fill in the fields and press Calculate.
Percent error tells you how close an experimental measurement is to the accepted, true value — a staple of every science lab report. It expresses the gap as a percentage of the accepted value, so a small error on a large quantity and a large error on a small one can be compared fairly.
Enter your measured value and the accepted value; the tool returns the percent error, the signed error and the absolute difference.
How is it calculated?
The percent error formula
percent error = |measured − accepted| ÷ |accepted| × 100
| Result | Meaning |
|---|---|
| Percent error | The magnitude of the error, always positive |
| Signed error | Same value with a sign: positive if you over-measured, negative if under |
| Absolute difference | measured − accepted, without dividing |
Why divide by the accepted value
Dividing by the accepted (true) value turns the raw difference into a *relative* error, so it's comparable across scales. Being 1 cm off on a 1 m length (1%) is far better than being 1 cm off on a 5 cm length (20%), and the percentage captures that where the raw 1 cm doesn't.
Sign tells you the direction
The standard percent error uses absolute value, so it's always positive. But the *signed* error is often more useful: a positive sign means your measurement was too high, a negative sign too low. A consistent sign across repeats points to a systematic error (a mis-calibrated instrument) rather than random scatter.
Where it helps
Chemistry and physics lab reports, calibration checks, and comparing an estimate against a known figure. Percent error compares against a known true value; when there's no accepted value and you're comparing two measurements to each other, percent *difference* is used instead.
Worked example
You measure the density of aluminium as 2.60 g/cm³; the accepted value is 2.70 g/cm³. The percent error is |2.60 − 2.70| ÷ |2.70| × 100 = 0.10 ÷ 2.70 × 100 = 3.70%. The signed error is −3.70%, the minus sign showing you under-measured, and the absolute difference is 0.10 g/cm³.
FAQ
How do I calculate percent error?+
Take the absolute difference between the measured and accepted values, divide by the accepted value, and multiply by 100: |measured − accepted| ÷ |accepted| × 100.
Why divide by the accepted value?+
Dividing by the true value makes the error relative, so measurements on different scales can be compared. A 1 cm error is 1% on a metre but 20% on 5 cm.
Can percent error be negative?+
The standard percent error uses absolute value and is always positive. This tool also shows the signed error: positive means you measured too high, negative too low.
What is a good percent error?+
It depends on the experiment, but in a school lab under about 5% is usually considered good. Consistently large errors in one direction suggest a systematic problem with the method or instrument.
What is the difference between percent error and percent difference?+
Percent error compares a measurement to a known accepted value. Percent difference compares two measured values to each other when neither is the accepted truth.