How Do You Calculate the Length of a Stair Stringer?

Once you have found these measurements, multiply the riser measurement by itself and the tread measurement by itself, and add these two numbers. Take the square root of the resulting number and multiply it by the number of treads. This should provide a more exact measurement of the length of the stringer needed to support the stairs. Check building codes in your area to make sure of the maximum riser height and the minimum tread width.