The U.S. National Building Code has the following specific requirements for critical safety dimensions for rotating staircases:
Minimum clear width: typically 660 mm (26 inches).
Minimum tread depth: typically 190 mm (7.5 inches) at 305 mm (12 inches) from the narrower edge.
Maximum kick surface height: typically 241 mm (9.5 inches).
Minimum clear height: typically 1982 mm (6 feet 6 inches).
In addition, the 2018 IBC specifies a minimum clear tread depth of 171 mm (6.75 inches) and a minimum clear height of 1981 mm (78 inches) at 305 mm (12 inches) from the narrower edge. According to the IRC, all treads should be identical and have a travel line radius no greater than 622 mm (24.5 inches).