The stair tread is the surface area of each step within the stair assembly. The distance between each step is referred to as the rise. Both factors contribute to the safety and comfort of the people who use the stairs. The tread size is calculated based on the horizontal distance between the front edge of the top landing and the planned location of the base of the stairs. This distance is divided by the number of planned steps. While the actual size is determined by calculation, there are minimum standards and common averages builders can reference.
The minimum tread size is commonly stated as 9 inches for residential construction. Harvard University lists typical tread sizes between 8 and 11 1/2 inches. However, the Americans with Disabilities Act (ADA) lists the minimum tread size as 11 inches, which would be the effective minimum and likely the average size of the tread for any stairs built for public use.
The width of the tread from side to side is commonly stated as a minimum of 36 inches for residential construction and 48 inches for ADA compliant public buildings. Both sizes accommodate wheelchairs and other implements.
The rise of each stair also differs between residential and ADA compliant construction. Residential risers commonly vary from 6 to 9 inches per step. Commercial construction has a maximum rise per step of 7 inches to remain ADA compliant. Basically steps in an ADA compliant building will be lower with wider treads and a wider stairway than residential construction.
The Occupational Safety and Health Administration (OSHA) requires the rise and tread of each stair to be within 1/4 inch in size of the rest. This consistency makes people using the steps less prone to stumbling and falling. While OSHA regulations do not apply to residential construction, you should aim to be as consistent as possible in the size of each step.