Step 1
Check local building codes to determine maximum and minimum ranges for tread depth and riser height. Typically, treads must be about 10 inches deep and risers no taller than 7 3/4 inches. However, those dimensions can vary across the nation, so checking beforehand will save potential rework later on.
Step 2
Measure from the top of the finished second story flooring to the finished first story flooring. Keep the tape measure plumb, or perfectly vertical, while taking this measurement. Record it with a pencil on a pad of paper. For this example, assume the measurement is 108 inches. Floor to ceiling is usually about 96 inches, plus the width of second-floor framing lumber, and the thicknesses of first floor drywall and second-story flooring materials.
Step 3
Divide the rise by the height of the risers. Use a calculator to make the math easier. In the example, divide 108 by 7 3/4 for a quotient of 13.93. Round the quotient to the nearest whole riser; in this case, 14. It will take 14 risers to reach from the first floor to the second in this example. Note that the 14th riser is topped by the second story flooring. In addition to 14 risers, the staircase will have 13 treads, since the final, 14th tread, is actually second story flooring.
Step 4
Calculate the run of the staircase by multiplying the width of the treads--10 inches in this example staircase--by the number of treads in the staircase: 13. So, the example staircase covers a run of 130 inches.