Used to describe the spread of the distribution of numbers. Standard deviation is calculated by the following steps:
a) determine the mean (average) of a set of numbers.
b) determine the difference of each number and the mean
c) square each difference
d) calculate the average of the squares
e) calculate the square root of the average.

You can also use Excel function STDDEVPA to calculate standard deviation. In safety stock calculations the forecast quantity is often used instead of the mean in determining standard deviation.