Stairs are a difficult part of any building or remodelling process. If stairs are built improperly, they can cause accidents. Treads that are too short or uneven riser heights make stairs clumsy and dangerous to use. There is a basic formula to calculate the treads and risers for stairs. The math is fairly simple, but a calculator makes rechecking quick and painless. The formula uses standard step measurements that are adequate for most building codes, but ask the local building code enforcement agency if there are any special requirements for stairs in your area.
Measure the total stair height with a measuring tape, from the bottom of where the stairs will reach to the top of the finished floor above. Make note of the measurement, in inches.
Divide the height by 7, which is a standard approximate riser height in inches. Round the answer (up if the decimal place is .5 or greater, down if it's under .5). This gives the number of risers.
Divide the height in inches by the number of risers you determined in the previous step. This gives the exact riser height, in inches.
Subtract one from the number of risers to get number of treads. Multiply the number of treads by 10.25, which is the ideal tread width in inches. This gives the total run of the stairs. Ensure that you have enough space to build a staircase with a run of this length.
When the stringers are made, make sure to make the top tread shorter than the other tread by the thickness of the finished floor. For outdoor projects, this is usually 3/4 of an inch.