The four numbers that fix the geometry
A spiral stair is set by the floor-to-floor height, the tread count, the outer diameter and the center pole. Everything else follows from those. Rise and rotation are the two you get wrong most often.
- Rise — equal on every tread; the IRC caps it at 9.5 inches. A 9-foot (108 in) floor over 13 treads gives 8.3 inches, which is comfortable.
- Center pole — the inner column all treads bolt to. Its diameter sets the inner edge; a 3.5 to 5 inch pole is typical for residential kits.
- Walkline — the design path, two-thirds of the way from pole to outer edge. Code dimensions are checked here, not at the narrow inner end.
Tread depth is measured on the walkline, not the wide end
Each tread is a pie slice, so it is deep at the rim and pinched at the pole. The depth that counts is on the walkline. It equals the tread's turn angle (in radians) times the walkline radius, so more treads per turn or a bigger diameter both deepen the step.
Walkline radius = center-pole radius + ⅔ of the remaining run to the outer edge, equal to (inner + outer diameter) ÷ 4. The IRC requires at least 6.75 inches of tread here and caps the walkline radius at 24.5 inches from center.
The turn eats your headroom
Headroom is the trap. As the stair winds up, the treads above hang over the ones below, so the tightest clearance is one full turn overhead — not at the entry. Code wants 78 inches (6 ft 6 in) at every point of the spiral.
Size the ceiling opening for the full outer diameter plus the 78 inch clearance, or the person one turn up hits their head.
Common questions
How do I find the rotation per tread on a spiral stair?
Divide the total rotation by the number of treads. A stair that turns 360 degrees over 12 treads rotates 30 degrees per tread. Most single-story spirals use 270 to 360 degrees of total turn.
Where is tread depth measured on a spiral stair?
On the walkline, the path a person actually walks, set two-thirds of the way out from the center pole to the outer edge. Depth there must be at least 6.75 inches under the IRC, and the walkline itself cannot sit more than 24.5 inches from center.
How much headroom does a spiral stair need?
At least 6 feet 6 inches (78 inches) measured vertically at the walkline, and it has to hold at every point of the turn, not just the entry. As a quick check, the ceiling opening should clear the outer diameter plus that 78 inches.
