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Spiral Staircase Calculator

Plan a spiral stair around a centre pole: how many risers, how far it turns, how deep the treads are where you walk, and whether the headroom works.

Spiral stair

Units
9 1/2 in is the IRC maximum for spiral stairs.
Outside diameter of the treads.
30° gives 12 treads per full turn.
More options
Width the handrail takes from the clear width.

12 risers of 9 in, 11 treads turning 330°, 7 5/16 in at the walkline, headroom 8 ft 10 1/2 in. Results and details

Worked example: edit any value

Spiral layout

Risers 12
Riser height 9 in
Tread at walkline ≈ 7 5/16 in
Headroom 8 ft 10 1/2 in one full turn

Treads 11 Rotation 330° Clear width 28 in

Spiral stairs are usually bought as kits sized by the maker. Use this to check a kit or plan the opening; follow the manufacturer for structure and the local code for approval.

Spiral stair planPlan view: 11 treads of 30 degrees around a 4 in pole, 5 ft 0 in diameter; walkline 14 in from the centre.1234567891011walkline (dashed) 14 in from centre · tread there 7 5/16 in5 ft 0 in diameter · 11 treads × 30° = 330°
Plan view, to scale. Numbers are treads; the shaded sector is the top platform; the dashed circle is the walkline.

Informational checks (IRC 2024 R318.7.11.1)

  • Within referenceClear width. 28 in against a reference minimum of 26 in. IRC 2024 R318.7.11.1
  • Within referenceWalkline radius. 14 in against a reference maximum of 24 1/2 in. IRC 2024 R318.7.11.1
  • Within referenceTread at the walkline. 7 5/16 in against a reference minimum of 6 3/4 in. IRC 2024 R318.7.11.1
  • Within referenceRise. 9 in against a reference maximum of 9 1/2 in. IRC 2024 R318.7.11.1
  • Within referenceHeadroom. 106 1/2 in (one full turn, less tread thickness) against a reference minimum of 6 ft 6 in. IRC 2024 R318.7.11.1
How this result was calculated
  1. Risers and treadsR = ⌈108 ÷ 9.5⌉ = 12; h = 9 in; treads = R − 1 = 11The top platform is the last step.
  2. Rotation11 × 30° = 330°
  3. Tread at the walkliner = pole radius + 12 in = 14; 2πr × 30/360 = 7.3304 inThis tool places the walkline 12 in out from the pole; the IRC also caps the walkline radius at 24 1/2 in.
  4. Headroom one full turn up(360 ÷ 30) × 9 − 1.5 = 106.5 in
All values (table)
ItemValueNote
Risers / treads12 / 11top platform is the last step
Riser height9 in9 in
Tread angle30°12 treads per turn
Total rotation330°
Clear width28 inouter radius − pole radius − handrail
Tread at walkline≈ 7 5/16 inat 14 in from centre
Tread at outer edge≈ 15 11/16 in
Headroom8 ft 10 1/2 inone full turn

Spiral stairs have their own rules

The IRC treats spiral stairs separately (2024 R318.7.11.1): clear width at least 26 in, walkline radius at most 24 1/2 in, each tread at least 6 3/4 in deep at the walkline, identical treads, risers at most 9 1/2 in, and headroom at least 6 ft 6 in. These are steeper and tighter than a straight stair is allowed, which is why spiral stairs save so much space.

This tool places the walkline 12 in out from the pole (the IRC's walkline offset for turns, R318.7.4) and reports that radius so you can compare it with the 24 1/2 in cap. Manufacturers may define the walkline differently for a kit; follow their data for a kit.

Worked example

A 108 in rise, 60 in diameter, 4 in pole, 30° treads, 1 1/2 in thick treads:

  • 108 ÷ 9.5 = 11.4, so 12 risers of 9 in; 11 treads plus the top platform; 11 × 30° = 330° of turn.
  • Clear width 30 − 2 = 28 in (26 in minimum).
  • Walkline at 14 in from the centre: tread depth 2π × 14 × 30 ÷ 360 = 7.33 in (6 3/4 in minimum).
  • Headroom one turn up: 12 × 9 − 1.5 = 106.5 in, well over 78 in.

These are automated test fixtures (spiral S1).

Questions people ask

These are questions Google showed under "People also ask" for related searches (checked 2026-10-24), answered here with the same tested formulas the calculators use.

How much space is required for a spiral staircase?

The footprint is a square the size of the stair diameter, plus room to step on and off. The IRC minimum clear width of 26 in (2024 R318.7.11.1) sets the floor: with a 4 in centre pole, the treads alone need at least 2 × (26 + 2) = 56 in of diameter before any handrail allowance. The floor opening above must be at least the stair diameter.

What is the rule for spiral staircases?

Under the IRC (2024 R318.7.11.1): clear width at least 26 in; walkline radius at most 24 1/2 in; each tread at least 6 3/4 in deep at the walkline; all treads identical; rise at most 9 1/2 in; headroom at least 6 ft 6 in. Spiral stairs are exempt from some general stair rules, and local codes may restrict where one can be the only stair.

How to calculate a spiral staircase?

Divide the total rise by the maximum riser and round up for the riser count; treads are one fewer because the platform is the top step. Choose a tread angle (30° gives 12 treads per turn); rotation = treads × angle. Check the tread depth at the walkline (2πr × angle ÷ 360) and the headroom one full turn up ((360 ÷ angle) × riser − tread thickness). The calculator above does all of this.