Residential stairs are governed by four numbers most builders can recite: 7-3/4, 10, 36, and 6-8. Miss the uniformity tolerance, though, and a stair that meets all four still fails.
Largest riser or tread may not exceed the smallest by more than 3/8 in.
IRC R311.7.5.1, R311.7.5.2
Minimum width
36 in. clear above the handrail height; 31.5 in. at/below one handrail, 27 in. between two
IRC R311.7.1
Headroom
6 ft 8 in. minimum, measured vertically from the nosing line
IRC R311.7.2
Nosing
3/4 in. to 1-1/4 in. projection required where treads are less than 11 in. (solid risers)
IRC R311.7.5.3
Landings
Landing or floor required at top and bottom; minimum 36 in. in the direction of travel
IRC R311.7.6
Riser openings
Open risers may not pass a 4-in. sphere where the stair is more than 30 in. above grade/floor
IRC R311.7.5.1
Worth knowing
The commercial (IBC) standard is stricter — 7 in. max rise, 11 in. min tread — which is why "code stairs" answers online conflict.
Spiral stairs have their own allowances: 26 in. clear width, 6-3/4 in. tread at the 12-in. walkline, 9-1/2 in. max rise (R311.7.10.1).
The 3/8-in. uniformity rule is the most common stair failure — check the first and last riser after flooring changes.
Winder treads need 10 in. at the 12-in. walkline and 6 in. minimum at any point.
This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org. Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.