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Stairs Calc

Tread angle (rotation)

The plan-view angle each spiral or winder tread sweeps around the center.

On a spiral or winder stair, the tread angle is how many degrees each step sweeps around the center column in plan view. Multiply it by the number of treads to get the total rotation. Example: a spiral staircase with 12 treads at 30° each rotates 360° — exactly one full turn from bottom to top. A wider sweep per tread means fewer, deeper-feeling steps but a tighter helix; a narrower sweep gives a gentler climb over more rotation. The angle is geometry, not code, but it interacts with the walkline depth and headroom limits, since a faster rotation can drop the diagonal clearance under the step above.

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Related terms

Spiral staircase A helical stair whose wedge treads wind around a central column. Walkline The path 12 in in from the inside of a turn, where winder tread depth is measured. Stair angle (pitch) The slope of the flight in degrees; comfortable stairs sit around 30–37°. Headroom Vertical clearance measured plumb above the nosing line; minimum 6 ft 8 in (IRC).

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Spiral Staircase Calculator Enter your floor‑to‑floor height and diameter to get tread count, tread angle, rotation and headroom — with the IRC spiral‑stair limits checked automatically.
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Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.

Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.

Last reviewed 2026-06-20 against IRC 2021/2024

Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.