If this is a population:

 1. Put all the numbers in a vertical column 
 2. Find the sum by adding all the numbers.
 3. Count them to find the number of numbers.
 4. Divide the sum by the number of numbers.
 5. That's the mean, denoted by μ
 6. Subtract μ from each number, and list 
    these values in a new vertical column out 
    to the right of the first one.
 7. Square each number in that column, and
    list their values in another vertical
    column out to the right of the others.
 8. Find the sum of that column by adding that 
    column.
 9. Divide that sum by the number of numbers. 
    That's the population variance, written 
    as σ².
10. Take the square root of the population 
    variance. That's the population standard 
    deviation, written as σ.

[If the numbers represents a SAMPLE taken from a POPULATION,
 and not the entire population, then do the same through 
step 8.  Then: 

 9. Divide that sum by ONE LESS THAN the number of numbers. 
    That's the SAMPLE variance, written as s².
10. Take the square root of the sample variance. That's
    the sample standard deviation, written as s.

Edwin