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Stairs Calc

Walkline

The path 12 in in from the inside of a turn, where winder tread depth is measured.

The walkline is the imaginary path people actually walk along a turning staircase, taken 12 in (305 mm) in from the inside edge of the turn or handrail. It matters for winder treads, which are wedge-shaped and narrow toward the center: code measures their minimum depth at the walkline, not at the skinny inner tip. Example: an IRC winder must give at least a 10 in run at the walkline and never less than 6 in at the narrowest point. UK Approved Document K measures the going at two lines, 270 mm from each end. Designing to the walkline keeps a turning stair as safe to climb as a straight one.

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Related terms

Winder A wedge-shaped tread that turns a staircase without a flat landing. L-shaped stairs A staircase with a 90° turn, made with a quarter landing or winders. Run (going) The horizontal depth of one step, measured nosing-to-nosing, not counting the overhang. Spiral staircase A helical stair whose wedge treads wind around a central column.

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L‑Shaped & U‑Shaped Staircase Calculator Design quarter‑turn (L), half‑turn (U) and winder stairs — Stairs Calc sizes each flight, lays out the turn, and checks winder treads at the walkline.
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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.