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- Compares a measured value with the true or accepted value.
- Percent error = |measured − true| ÷ |true| × 100.
- Also shows absolute error, relative error, signed error and accuracy.
- Used in chemistry and physics labs, surveying, forecasting and quality checks.
How to use the percent error calculator
Enter the measured value
This is the number you got from your experiment, reading or estimate.
Enter the true value
This is the accepted, theoretical or reference value you are comparing against.
Choose decimal places (optional)
Open the rounding option to show the answer with 0 to 4 decimal places.
Read the result
You get the percent error, the absolute and relative error, and a short note saying whether you overestimated or underestimated.
How percent error is calculated
Percent error formula
Percent error = |measured − true| ÷ |true| × 100
Absolute error = |measured − true|
Relative error = absolute error ÷ |true|
The vertical bars mean absolute value, so percent error is never negative. The signed percent error keeps the sign to show the direction of the miss.
Because the formula divides by the true value, percent error cannot be calculated when the true value is 0. In that case report the absolute error instead. Percent error is different from percent difference, which compares two measured values with each other and divides by their average rather than by a known true value.
Example 1: measuring gravity
A student measures the acceleration due to gravity as 9.6 m/s². The accepted value is 9.81 m/s².
- Measured − true 9.6 − 9.81 = −0.21
- Absolute error 0.21
- Relative error 0.21 ÷ 9.81 = 0.0214
- Percent error 2.14%
The measurement was 2.14% too low, which is a reasonable result for a school pendulum experiment.
Example 2: weighing a sample
A scale shows 52 g for a reference weight of exactly 50 g.
- Absolute error |52 − 50| = 2 g
- Percent error 2 ÷ 50 × 100 = 4.00%
- Signed percent error +4.00% (overestimate)
A 48 g reading would also give 4.00% percent error, but the signed error would be −4.00%.
Typical accuracy targets
Rough guides only. Your teacher, lab or industry standard decides what counts as acceptable.
- School lab experiments often within 5% to 10%
- University lab work often within 1% to 5%
- Calibrated instruments often well under 1%
Smaller percent error means the measurement is closer to the accepted value.
Where a percent error calculator helps
Science lab reports
Report how close your result came to the textbook value and explain sources of error.
Engineering and quality checks
Compare measured parts or sensor readings against their specification.
Forecasts and estimates
See how far a budget, sales or time estimate was from what really happened.
Checking instruments
Test a scale, thermometer or meter against a known reference value.
Percent error always measures against a known true value. If you only want to know how much a number went up or down, use the percentage calculator. To average several trial results before finding the error, try the average calculator, and use the rounding calculator to report the answer to the right precision.
Percent error calculator FAQ
Can percent error be negative?
Standard percent error uses absolute values, so it is always 0 or positive. The signed percent error shown below the result can be negative and tells you the measurement was too low.
What is a good percent error?
It depends on the task. Many school labs accept under 5% to 10%, while precise lab or industrial work may need well under 1%.
Why can the true value not be 0?
The formula divides by the true value, and dividing by 0 is undefined. Use the absolute error in that case.
What is the difference between percent error and percent difference?
Percent error compares a measurement with a known true value. Percent difference compares two measurements when neither is known to be correct.
Can percent error be more than 100%?
Yes. If the measured value is more than double the true value, or far off in the other direction, the error can exceed 100%. That usually points to a unit or reading mistake.
How many decimal places should I report?
Match the precision of your data. Two decimal places is common, and you can change it in the rounding option.