Stairs are made up of tread, which is the horizontal part of the stair you step on, and risers, which are the verticals holding up the tread. Laws regulate the height of risers, and require risers to be of uniform height.
The height of stair risers should be 7.75 inches or less. Risers must be of uniform height, with a variation of 3/8-inch or less. Open risers are permitted if they are 4 inches high or less.
Calculating Rise and Run
To calculate the rise, or height, of a stairway, measure the height from one floor level to the next. Divide that total by 7.75 inches to see how many stairs you will need. Multiply the number of stairs by the expected width of each tread or step to determine the run, or the length of the stairway.
Local building codes may specify other requirements for stairs, including rules regarding amount of headroom, and placement and dimension of stair rails.