The formula for standard deviation is:
where x represents each of the data elements, x-bar represents the average of the x-values, and n represents the number of x-values.
To do this by hand:
1) find the average of all the x-values
2) for each x-value, find the difference between the x-value and the average value, and then square that answer
3) sum all the values from step 2
4) divide the answer of step 3 by n-1, the number of values minus 1
5) find the square root of step 4.
I think that is standard deviation of a sample
For standard deviation of a population I think that you use n instead of n-1