Percent Error Calculator
Percent error is the percentage ratio of the observed value and the true value difference over the true value.
Observed Value  
True Value  
In the real world, the data measured or used is normally different from the true value. The error comes from the measurement inaccuracy or the approximation used instead of the real data, for example use 3.14 instead of π. Normally people use absolute error, relative error, and percent error to represent such discrepancy:
absolute error = V_{true}  V_{used}
relative error = (V_{true}  V_{used})/V_{true} (if V_{true} is not zero)
percent error = (V_{true}  V_{used})/V_{true} X 100 (if V_{true} is not zero)
Where:

V_{true} is the true value
V_{used} is the value used
The definitions above are based on the fact that the true values are known. In many situations, the true values are unknown. If so, people use the standard deviation to represent the error. Please check the standard deviation calculator.