Question: what is the standard width of a stair tread?
Answer: The general rule (in the US) is 7-11 (a 7 inch rise and 11 inch run) (17.78cm-27.94cm). More exactly, no more than 7 3/4 inches (19.7cm) for the riser (vertical) and a minimum of 10 inches (25.4cm) for the tread (horizontal or step). You can find some more information here as well on other stair-related dimensions.