Find the average of your set of numbers. This can be done by adding all your numbers together, and dividing the sum by the number of values. For example, if you had four numbers in your list, you would divide the sum of all numbers by four.
Subtract the average from each number in the original list. These new values are called "deviations from the average."
Use a process called root-mean-square (RMS) to find the standard deviation. To start this process, square all of the deviations individually.
After each value has been squared, add all the new values together.
Divide the sum by the amount of numbers in the original list.
Take the square root of this number; this will be your standard deviation value.