Spiral staircase calculator: how many steps does a spiral staircase need?
12 risers
of 9 in · 11 treads at 30°, 7.33 in deep on the walkline · 330° of turn · for 108 in floor to floor
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
12 risers
of 9 in · 11 treads at 30°, 7.33 in deep on the walkline · 330° of turn
Source: 2021 International Residential Code, R311.7.10.1 Spiral stairways and R311.7.4 Walkline (2021 Seattle Residential Code, chapter 3, no Washington amendment to either), read October 24, 2026. Data sheet (PDF)
108 inches floor to floor takes a spiral staircase of 12 risers of 9 in, under the 9 1/2 in IRC 2021 R311.7.10.1 maximum; at 30° a tread around a 4 in column, treads are 7.33 in deep on the walkline.
A worked example
A loft 131 1/2 in above the floor, with a 66 in stair on a 6 in column turning 27° a tread needs 14 risers. 131 1/2 ÷ 9 1/2 is 13.8, so 14 risers of 9 3/8 in and 13 treads, turning 351° in all. The walkline sits 15 in from the center, where 27° gives 7.07 in of tread; the clear width is 30 in and one full turn climbs 125 in, over the 6 ft 6 in of headroom.
What does the code require for a spiral staircase?
IRC 2021 R311.7.10.1: clear width at least 26 in, walkline radius 24 1/2 in at most, treads at least 6 3/4 in deep at the walkline and all identical, risers 9 1/2 in at most and headroom at least 6 ft 6 in. The walkline runs 12 in out from the inside of the turn (R311.7.4).
How many treads does a spiral staircase have?
One fewer than the risers, the upper floor being the last step: 108 in floor to floor takes 12 risers of 9 in and 11 treads, which turn 330° at 30° each. Around a 4 in column a tread has to turn at least 27.7° to be 6 3/4 in deep on the walkline.
What else it is called
Also called a spiral stair or circular staircase; the walkline is the path a person walks, 12 in out from the center column, and the turn per tread is the angle between two treads. Searched as a spiral staircase calculator or spiral stair calculator.
Risers by floor to floor height
| Floor to floor | Risers | Riser height | Treads | Turn at 30° a tread | One full turn climbs |
|---|---|---|---|---|---|
| 96 in (8 ft 0 in) | 11 | 8 3/4 in | 10 | 300° | 104.7 in |
| 102 in (8 ft 6 in) | 11 | 9 1/4 in | 10 | 300° | 111.3 in |
| 108 in (9 ft 0 in) | 12 | 9 in | 11 | 330° | 108 in |
| 114 in (9 ft 6 in) | 12 | 9 1/2 in | 11 | 330° | 114 in |
| 120 in (10 ft 0 in) | 13 | 9 1/4 in | 12 | 360° | 110.8 in |
| 126 in (10 ft 6 in) | 14 | 9 in | 13 | 390° | 108 in |
| 132 in (11 ft 0 in) | 14 | 9 7/16 in | 13 | 390° | 113.1 in |
| 144 in (12 ft 0 in) | 16 | 9 in | 15 | 450° | 108 in |
Risers of 9 1/2 in at most, all identical (IRC 2021 R311.7.10.1); treads = risers − 1, the upper floor being the last step. On a 4 in center column the walkline sits 14 in out, where 30° gives a 7.33 in tread and 27.5° a 6.72 in tread, under the 6 3/4 in minimum.
Where the numbers come from
- Spiral stairs: IRC 2021 R311.7.10.1, clear width «not less than 26 inches», walkline radius «not greater than 24-1/2 inches», each tread «not less than 6-3/4 inches» deep at the walkline, treads identical, rise «not more than 9-1/2 inches», headroom «not less than 6 feet 6 inches». The walkline is «located 12 inches (305 mm) from the inside of the turn» (R311.7.4). As printed in the 2021 Seattle Residential Code, chapter 3, and identical in the IRC 2021 on UpCodes (New Jersey). Read October 24, 2026. IRC 2021 chapter 3 (PDF)
- The rest is arithmetic: risers = floor to floor ÷ 9.5, rounded up; treads = risers − 1; walkline radius = column ÷ 2 + 12 in; tread depth there = 2π × radius × degrees ÷ 360; clear width = (diameter − column) ÷ 2; one full turn climbs 360 ÷ degrees × riser.
The headroom check is one full turn of risers measured from the walking surface, before the thickness of the tread above. The clear width is taken from the column to the outside of the stair, and a handrail set inside that line narrows it.
By Javier Miralles · Last updated . How this site is made.
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