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 22.5 cm (9 inches) for residential construction. Harvard University lists typical tread sizes between 20 and 28.7 cm (8 and 11 1/2 inches). For people with disabilities the minimum tread size is 27.5 cm (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 90 cm (36 inches) for residential construction and 1.2 m (48 inches) for disabled access to public buildings. Both sizes accommodate wheelchairs and other implements.
The rise of each stair also differs between residential and public building construction. Residential risers commonly vary from 15 to 22.5 cm (6 to 9 inches) per step. Commercial construction has a maximum rise per step of 17.5 cm (7 inches) to remain compliant. Basically, steps in a public building will be lower with wider treads and a wider stairway than residential construction.
The rise and tread of each stair should be within 6 mm (1/4 inch) in size of the rest. This consistency makes people using the steps less prone to stumbling and falling. You should aim to be as consistent as possible in the size of each step.