The standard deviation of a random variable, sample, statistical population, data set, or probability distribution is the square root of its variance. (For a finite population, variance is the average of the squared deviations from the mean.)
To calculate standard deviation, start by calculating the mean, or average, of your data set. Then, subtract the mean from all of the numbers in your data set, and square each of the differences.
The standard deviation is the average amount of variability in your dataset. It tells you, on average, how far each score lies from the mean.