- 1). 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.
- 2). 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 floor-to-floor 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 floor-to-floor measurement as 109.625 inches.
- 3). 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.
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