How to Figure the Height for a Stair Layout

Measure the total vertical rise of the stair from the finished level of the upper floor to the finished level of the floor at the lower level. For ease of mathematics, record all measurements in inches to the nearest 1/8 inch.

Measure the total run. This is the horizontal distance from a point directly under the edge of the upper floor to the point you plan for the stair to end.

Divide the total rise by the planned number of stairs. The rise of each stair must be equal and no more than 8 1/4 inches. For example, a stairway with a rise of 96 inches could have 12 steps with an 8-inch rise to each stair. A rise of 90 inches would have 12 steps of 7 1/2 inches or 11 steps of 8 1/8 inch.

Divide the total run of the stairs by the number of steps. This is the tread width of each step and should not be less than 9 inches. For example, if the total run is 120 inches and the number of steps is 12, each step would have a tread width of 10 inches. If the total run is 100 inches and 12 steps are planned, then each tread would be 8 1/3 inches wide and too narrow for safety.