How to Calculate Stair Risers
A properly designed staircase begins with some basic calculations to determine the length and height of each step, as well as the overall area needed for the stairs. The first of these critical preliminary measurements is the step height, commonly referred to as the "riser" height. Determining the necessary stair riser height also determines the number of threads and the overall length, known as the "run," of the staircase. The correct calculation of the risers will insure a wellbuilt, comfortable staircase for your home.

Measure the vertical distance, in inches, between the first floor and the second floor surfaces at the proposed stair location. You can use a plumb bob to make certain the measurement is accurately vertical.

Convert any fraction of an inch in the measurement to a decimal by dividing the bottom number, the denominator, into the top number, the numerator. For example, if the floortofloor measurement is 109 5/8 inches, the fraction is 5/8 inch. Divide 8 into 5 for the decimal equivalent of .625, which translates the floortofloor measurement as 109.625 inches.

Divide the measurement determined in Steps 1 and 2 by a number that gives you an equal riser height between 7 inches and 8 inches each. For our example, the result for 109 5/8 inches is 14 risers at 7 13/16 inches each or 15 risers at 7 5/16 inches. Typically, the number of risers chosen will be influenced by the resulting stair run.
Things You Will Need
 Tape measure
 Plumb bob (optional)
 Calculator
Tips
 The run of the stairs can be determined by multiplying the tread width times the number of risers determined minus one riser.
 National and local building codes typically specify the limits for residential riser heights between 7 inches and 8 inches. Consult your local building department to verify the code requirements for your area.