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Stairs Calc

Spiral staircase

A helical stair whose wedge treads wind around a central column.

A spiral staircase winds its wedge-shaped treads helically around a central column, occupying the smallest footprint of any stair type. Because every tread is a winder, code measures depth at the walkline — the IRC requires at least 6¾ in (171 mm) of tread depth there, allows a riser up to 9½ in (241 mm), and reduces headroom to 6 ft 6 in (1982 mm). Example: a 60 in (1524 mm) diameter spiral with a 96 in floor-to-floor rise needs roughly 13 treads and just over a full 360° turn. The trade-off for the tiny footprint is that spirals are awkward for moving furniture and are often restricted to secondary access.

Try the calculator Spiral Staircase Calculator

Related terms

Winder A wedge-shaped tread that turns a staircase without a flat landing. Walkline The path 12 in in from the inside of a turn, where winder tread depth is measured. Headroom Vertical clearance measured plumb above the nosing line; minimum 6 ft 8 in (IRC). Tread angle (rotation) The plan-view angle each spiral or winder tread sweeps around the center.

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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.