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 well-built, comfortable staircase for your home.

Accurate calculation of step risers is key to a well-constructed stairway.
  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.

About the Author

Paul Massey has been writing since 2009, drawing on a 35-year career in the construction industry. His experience includes 15 years as a general building contractor specializing in architectural design, custom homes, commercial development and historic renovations.