From the 2018 International Residential Code unless otherwise indicated.
R311.7.1 - Width
Stairways shall not be less than 36 inches (914 mm) in clear width at all points above the permitted handrail height and below the required headroom height. The clear width of stairways at and below the handrail height, including treads and landings, shall not be less then 31.5 inches (787 mm) where a handrail is installed on one side and 27 inches (698 mm) where handrails are provided on both sides.
R311.7.2 - Headroom
The headroom in stairways shall not be less than 6 feet, 8 inches (2032 mm), measured vertically from the sloped line adjoining the tread nosing or from the floor surface of the landing or platform on that portion of the stairway.
R3188.8.131.52 - Risers
The riser height shall not be more than 7 3/4 inches (196 mm).
R3184.108.40.206 - Treads
The tread depth shall not be less than 10 inches (254 mm).