Stair stringers are the support boards that hold the treads. The treads are the boards your feet come into contact with as you climb the stairs. The overall height of the stair is called the rise. The rise will vary depending on the application of the stairs. Outdoor installations may have an uneven landing. Indoor stairs will most typically use a landing of the wooden floor. Regardless of the landing, it must be wide enough to support the stringer and be perfectly level.
Measure the overall height difference between the finished floor of the upper portion where the stairs are leading and the ground or ground floor. This measurement is the rise of the stairs.
Use the pencil and paper. Divide the riser height from this overall rise to find the number of risers, treads, to be used. The riser height is the height of each individual stair. International Building Codes (IBC) set this riser height at 7 ¾ inches. Local codes may limit this to 7 inches. The condition of your climbing ability may limit it even further to a height of 6 inches. Check with local building codes.
Round up the number of risers, treads, from the answer found in Step 2. In other words if the amount of risers come out to be 9.4 risers. Round that up to 10 total stair risers.
Equalize the stair tread spacing by dividing the total amount of risers by the overall rise. As an example, if the rise is 68 inches divide that by 10 risers. The riser height would then be 6.8 inches or 6 7/8 inches between each riser or tread surface. This will create an equal stair for each tread.
Deduct the stair tread height from the bottom of the stringer. The stair tread height is the thickness of the board that will make up the tread. In other words if you are using a 2 by 10 board for the tread. The thickness of this board will be approximately 1-½ inches. The bottom of the stringer that rests on the bottom floor or outdoor pad will have to be removed, 1-½ inches, for even spacing from the ground.