Percent Error Calculator: Measured Versus True Value
Work out the percent error between a measured value and the accepted true value — the headline accuracy metric in chemistry labs, physics experiments, and engineering tolerances.
Adjust the inputs and select Calculate for a full breakdown.
Compare Common Scenarios
How the numbers shift across typical situations for this calculator:
| Scenario | Percent error | Measurement deviation |
|---|---|---|
| 100 true · 95 measured | -5.00% | -5 |
| 50 true · 53 measured | 6.00% | 3 |
| 9.81 true · 9.74 measured | -0.71% | -0.07 |
| 200 true · 210 measured | 5.00% | 10 |
How This Calculator Works
Enter the true (accepted) value and the measured value. The calculator subtracts true from measured for the deviation and divides by the true value for the percent error. A positive sign means the measurement is high; a negative sign means low. Take the magnitude as the error in either direction.
The Formula
Percentage Change
Old is the starting value, New is the ending value
Worked Example
A measurement of 95 against a true value of 100 produces a -5% percent error — the measurement is 5% below the accepted value. In most lab reports the figure is quoted as 5% (the magnitude), with the direction noted separately or implied by context.
Key Insight
Percent error scales the deviation against the true value, which keeps the figure comparable across measurements of very different magnitudes. A 0.01g deviation is huge on a 0.05g sample (20% error) and trivial on a 100g sample (0.01% error). The percent error is what to chase; absolute deviation in isolation can mislead.
Frequently Asked Questions
How is percent error calculated?
Subtract the true value from the measured value, divide by the true value, and multiply by 100. A measurement of 95 against a true value of 100 produces a -5% percent error.
Why use the true value in the denominator?
It anchors the error to the reference point, making percent errors comparable across measurements of any magnitude. A 1-unit error on a 10-unit true value is 10%; on a 1,000-unit true value it is 0.1%.
Should the sign matter?
It matters in metrology and quality control, where a systematic bias (consistently high or low) is itself diagnostic. Many lab reports quote magnitude only and note direction separately.
What is a good percent error?
It depends entirely on the field and instrument. Analytical chemistry typically targets under 5% for student labs and under 1% for professional ones; engineering tolerances can be much tighter or looser depending on the application.
How does percent error differ from percent change?
Same formula, different semantics. Percent change measures movement between two values where neither is 'right'; percent error measures deviation from a value treated as the reference truth.
Related Calculators
Methodology & Review
Percent error is the measured value minus the true value, divided by the true value, expressed as a percentage. Sign indicates direction: positive means the measurement is over the true value; negative means under. Many texts use the absolute value — read the magnitude here as the error in either direction.
Written by Ugo Candido · Last updated May 17, 2026.