Stair tread width versus height is regulated in the United States by the National Building Code, last updated in 1993. The code lists specifications for width and height of stair tread. However, local specifications may override the dimensions, so it is important to check with your local building authority before building or remodeling a staircase. Specifications differ for residential and commercial staircases. The NBC has instituted these measurements to help prevent accidents due to staircases being too small to solidly accommodate a person's foot. Should your stairs be covered with carpeting or tiles, it is imperative that the covering is securely fastened.
For residential staircases, the National Building Code has set the maximum riser height at 8-1/4 inches. The riser is the back of the step that makes it "tall," not the part you walk on. Making your steps less than 8-1/4" tall is acceptable and may be beneficial if there are elderly or disabled people living in the home.
The tread is the part of the step that your foot steps on as you ascend or descend a staircase. The minimum tread depth is 9 inches, which allows enough for a solid footing while traversing the stairs. Wider, or mixed size, treads are generally allowed on spiral staircases or staircases with a landing and twist. A tread depth of more than 9 inches is acceptable, especially if the staircase will be used by elderly or disabled people. Deep treads can also provide an elegant-looking staircase.
Some steps have a lip or "nose" which protrudes over the tread of the step. The building code allows no more than a 1-inch nose for such types of steps. More than 1 inch could cause someone's foot to become entangled on the tread.
The generally acceptable width of a stairway is 36 inches, including handrails. This is called the "clear width." The clear width allows the user to have a wide enough area to walk, and additionally allows people to pass each other on the steps safely.
One additional measurement that needs to be looked at is the headroom. The NBC has instituted a minimum height of 6 feet, 8 inches, which is measured from the landing to the ceiling directly above it. This height should be maintained throughout the staircase.
There are many types of staircases found in homes, including "floating" steps in which there is no visible riser. This type of staircase is still bound by the requirements of an 8-1/4-inch riser measurement. Spiral staircases, as well as those with multiple landings, allow for a variation in tread depth, but generally adhere to the same measurements for step risers and overall width.
For commercial buildings, the specifications are a bit different from residential. Generally, the stair tread should be 11 inches minimum, the riser height is specified between 4 and 7 inches, and the width should be at least 48 inches, excluding handrails. The width allows for a wheelchair user to be safely carried down in the event of an emergency.