What is the minimum width for a staircase?
Residential stairways must be a minimum of 36 inches (914 mm) in clear width, measured above the handrail height and below the required headroom (2018 IRC §R311.7.1). Handrails may project up to 4½ inches into that width on each side, so the clear width can narrow to 31½ inches with one handrail, or 27 inches with handrails on both sides.
Source: 2018 IRC §R311.7.1 · Verify against your AHJ and the adopted code edition.
The IRC sets the minimum stair width at 36 inches (914 mm), measured in the clear above the handrail and below the required headroom. That’s the number most people mean by “how wide do my stairs need to be.”
The catch is the handrail. A handrail is allowed to project up to 4½ inches into the stairway on each side, so the usable clear width below the handrail is smaller:
- One handrail: at least 31½ inches clear.
- Handrails on both sides: at least 27 inches clear.
So a 36-inch rough opening is the practical floor for a code stair — go narrower and you can’t keep 27–31½ inches once the rails and finished wall go on. The IBC requires more for most commercial stairs (typically 44 inches, or 36 inches where the occupant load is under 50). Editions and local amendments vary — verify with your AHJ.
Work it out: the stair calculator sizes the risers, treads, and total run so you can lay the flight into a 36-inch (or wider) opening.
Sources
- ICC — International Codes (IRC) — 2018 IRC §R311.7.1