Find your students' grades. For example, assume you have 3 students with grades of 89, 95 and 80.
Calculate the average grade by adding the grades together and dividing by the number of students. In the example, 89 plus 95 plus 80 equals 264. Then 264 divided by 3 equals 88.
Subtract the average from each grade, then square the result. This is called the square of the mean difference. In the example, 89 minus 88 equals 1, then 1 squared equals 1. Then 95 minus 88 equals 7, and 7 squared equals 49. Finally, 80 minus 88 equals -8, and -8 squared equals 64.
Add together the squares of the mean difference. In the example, 1 plus 49 plus 64 equals 114. This is your numerator.
Subtract 1 from the number of students you have. In the example, 3 minus 1 equals 2. This is your denominator.
Divide the numerator by the denominator to find your variance. In the example, 114 divided by 2 equals 57.
Take the square root of your variance to calculate your standard deviation. In the example, the square root of 57 equals a standard deviation of 7.5498.