Spiral Staircase Calculator

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Construction

Calculate number of steps, rotation per step, and tread width for a spiral staircase.

Formula

Steps = ⌈total rise / riser height⌉; Rotation = 360° / steps; Tread@walkline = 2π × walkRadius / steps
  • IRC §R311.7.10.1: minimum tread 6.67 in at walk line (12 in from narrower edge); min clear width 26 in.
  • Walk line radius = (diameter/2) − centre post diameter.
  • Maximum riser = 9.5 in; minimum headroom = 78 in (IRC §R311.7.2).

108 in rise, 7.5 in riser, 60 in diameter, 4 in post

Inputs
  • Total Rise: 108 in
  • Riser Height: 7.5 in
  • Overall Diameter: 60 in
  • Centre Post Diameter: 4 in

Steps = ⌈108/7.5⌉ = 15. Rotation = 24°/step. Walk radius = 26 in. Tread = 2π×26/15 = 10.89 in.

Frequently asked questions

What is the walk line?
The walk line is an imaginary line 12 inches from the narrower (inner) edge of the tread. The IRC measures minimum tread width at this line.