Square the numbers within a set. This means that you multiply each number times itself. For example, if you have 3, 5 and 6, you would multiply 3 times 3 to get 9, 5 times 5 to get 25, and 6 times 6 to get 36.
Add the squares from step one. In this example, you would add 9, 25, and 36 to get 70.
Divide the sum of the squares from step two by the number of values in the set. In this case, there are 3 numbers in the set, so you would divide 70 by 3 to get 23.33.
Take the square root of the quotient from step three. The square root is the value that, when multiplied times itself, equals the original number. In this case, you would take the square root of 70 to get a quadratic mean of 8.37.