In statistics, the z-score is the number of standard deviations an observation or datum is above or below the mean. A positive standard score indicates a datum above the mean, while a negative standard score indicates a datum below the mean. It is a dimensionless quantity obtained by subtracting the population mean from an individual raw score and then dividing the difference by the population standard deviation; this conversion process is called standardizing, or nominalizing. Standardized scores are useful because they have the same meaning universally.
Click for the equation to find a z-score for any individual value.
Source: wikipedia.org/wiki/Standard_score