🎯 Percent Error Calculator
By ToolNimba Editorial Team · Updated 2026-06-19
Percent error = |measured - actual| / |actual| × 100.
This percent error calculator tells you how far a measured (experimental) value falls from the true (theoretical) value, expressed as a percentage. Enter your measured value and the accepted actual value, and it shows both the absolute error and the percent error straight away. It is the standard check for accuracy in chemistry, physics, and any lab experiment.
What is the Percent Error Calculator?
Percent error measures the accuracy of a measurement: how close your result came to the value that is considered correct. It compares the size of the difference between your measured value and the accepted (true) value against the true value itself, then scales that to a percentage. A small percent error means your measurement was close to the target; a large one means it was off.
The calculation always uses the absolute value of the difference, so percent error is reported as a positive number regardless of whether you measured too high or too low. The accepted value goes in the denominator, because the question being asked is "how big is the error relative to what the answer should have been". Using the accepted value (not your measured value) as the reference is what makes results from different experiments comparable.
Percent error is different from percent difference and percent change, which people often confuse. Percent error compares an experimental result to a known correct value. Percent difference compares two measurements when neither is taken as the "true" value, and divides by their average. Percent change compares a new value to an older starting value and keeps the sign to show direction. Pick the one that matches your question before you reach for a formula.
When to use it
- Reporting the accuracy of a chemistry lab result against the textbook or accepted value.
- Checking how close a physics measurement (such as the acceleration due to gravity) came to the known constant.
- Evaluating whether a sensor, scale, or instrument reading is within an acceptable tolerance.
How to use the Percent Error Calculator
- Enter your experimental (measured) value in the first field.
- Enter the theoretical (actual or accepted) value in the second field.
- Read the percent error and the absolute error from the results.
- A lower percent error means a more accurate measurement; check your method if it is high.
Formula & method
Worked examples
A pendulum experiment measures gravity as 9.74 m/s², against the accepted 9.8 m/s².
- absolute error = |9.74 - 9.8| = 0.06
- percent error = 0.06 / |9.8| × 100
- percent error = 0.0061224 × 100 = 0.61%
Result: Absolute error 0.06, percent error about 0.61%
A scale reads 105 g for an object whose true mass is 100 g.
- absolute error = |105 - 100| = 5
- percent error = 5 / |100| × 100
- percent error = 0.05 × 100 = 5%
Result: Absolute error 5, percent error 5%
A titration finds a concentration of 48 mM where the actual value is 50 mM.
- absolute error = |48 - 50| = 2
- percent error = 2 / |50| × 100
- percent error = 0.04 × 100 = 4%
Result: Absolute error 2, percent error 4%
Interpreting a percent error result (general guide)
| Percent error | Typical interpretation |
|---|---|
| 0% to 1% | Very accurate, excellent agreement with the accepted value |
| 1% to 5% | Good accuracy, acceptable for most school and routine lab work |
| 5% to 10% | Moderate error, review technique or instrument calibration |
| Above 10% | Large error, likely a method, reading, or calculation problem |
Percent error vs related percentage measures
| Measure | Compares | Denominator | Sign kept? |
|---|---|---|---|
| Percent error | Measured vs accepted (true) value | Accepted value | No (absolute) |
| Percent difference | Two values, neither taken as true | Average of the two | No (absolute) |
| Percent change | New value vs old starting value | Old value | Yes (direction) |
Common mistakes to avoid
- Dividing by the measured value instead of the actual value. The accepted (true) value always goes in the denominator. Dividing by your measured result changes the answer and is not standard percent error.
- Forgetting the absolute value. Percent error is reported as a positive number. Skipping the absolute value can give a negative result, which describes direction (like percent change) rather than accuracy.
- Multiplying by 100 twice or not at all. The ratio |measured - actual| / |actual| is a decimal fraction. Multiply by 100 exactly once to turn it into a percent.
- Confusing percent error with percent difference. Use percent error only when one value is the accepted truth. When comparing two equally trusted measurements, use percent difference, which divides by their average.
Glossary
- Measured (experimental) value
- The result you actually obtained from your experiment or instrument.
- Actual (theoretical) value
- The accepted or true value you are comparing against, often from a reference or known constant.
- Absolute error
- The size of the difference between the measured and actual values, |measured - actual|, in the original units.
- Percent error
- The absolute error expressed as a percentage of the actual value, a measure of accuracy.
Frequently asked questions
What is the percent error formula?
Percent error = |measured value − actual value| ÷ |actual value| × 100. You take the absolute difference between your measured and accepted values, divide by the accepted value, and multiply by 100 to get a percentage.
How do I calculate percent error?
Subtract the actual value from your measured value, take the absolute value of that difference, divide it by the actual value, and multiply by 100. This calculator does all four steps for you and also shows the absolute error.
Can percent error be negative?
No. Because the formula uses the absolute value of the difference, percent error is always reported as zero or a positive number. A negative sign would describe direction, which belongs to percent change, not percent error.
What is a good percent error?
It depends on the experiment, but as a rough guide a percent error under 5% is usually considered good for school and routine lab work, and under 1% is excellent. Errors above 10% suggest a problem with method or equipment.
What happens if the actual value is zero?
Percent error is undefined when the actual value is zero, because the formula would divide by zero. In that case report the absolute error instead, which this calculator still shows.
What is the difference between percent error and percent difference?
Percent error compares a measured value to a known accepted value and divides by that accepted value. Percent difference compares two values when neither is the accepted truth and divides by their average instead.