About the Spiral Staircase Calculator
This spiral staircase calculator lays out a spiral stair from the floor-to-floor height and the stair diameter. It finds the number of risers and treads, the exact rise per step, the total rotation from bottom to top, the tread depth at the walkline, the clear width and the headroom under the turn above — the numbers you need to order a kit or check a fabricator’s drawing.
It is meant for homeowners adding a loft, attic or deck access stair and for builders confirming that a compact spiral will pass inspection. Spiral stairs are allowed as a means of egress in US homes when they meet the dimensions in the International Residential Code, so the result includes a pass/fail check against those limits.
The checks use 2021 IRC Section R311.7.10.1: clear width at least 26 in, walkline radius no more than 24½ in, tread depth at least 6¾ in at the walkline (12 in from the narrow end), rise no more than 9½ in, and headroom at least 6 ft 6 in. Clear width is approximated as the stair radius minus the center column radius; handrails and local amendments can reduce or change it, so confirm with your building department.
With the default inputs, the treads (plus top landing) is 11. Change any value above to recalculate instantly.
How to use the spiral staircase calculator
- 1Measure finished floor to finished floor in inches.
- 2Enter the stair diameter and center column size from the kit or drawing.
- 3Set the tread angle (30° is the most common).
- 4Keep the maximum rise at 9.5 in unless a stricter local rule applies.
- 5Review the code check table and adjust diameter or angle until every line passes.
Formula and method
The number of risers is the floor-to-floor height divided by the maximum allowed rise, rounded up, and the actual rise is the height divided by that count so all steps are identical. The upper floor or landing forms the last step, so there is one fewer tread than risers, and the treads turn through treads × tread angle in total.
Tread depth is measured along the walkline, an arc 12 inches out from the narrow end of the tread (the edge of the center column), so it equals the arc length 2πr × θ/360. Headroom is the vertical distance from a tread to the underside of the tread one full turn above: the number of treads per 360° times the rise, minus the tread thickness.
- H
- Floor-to-floor height (in)
- Rmax
- Maximum riser height (in)
- c
- Center column diameter (in)
- θ
- Tread angle (degrees)
- t
- Tread thickness (in)
Worked examples
9 ft floor-to-floor, 60 in diameter, 30° treads
108 ÷ 9.5 = 11.4, so 12 risers of exactly 9 in and 11 treads turning 330°. The walkline is 14.25 in from center, giving a 7.46 in tread depth; 12 treads per turn × 9 in − 1.5 in leaves 106.5 in of headroom.
10 ft floor-to-floor, 66 in diameter, 6 in column
120 ÷ 9.5 = 12.6 → 13 risers of 9.23 in. The 12 treads make one full 360° turn, the walkline radius is 15 in (7.85 in tread depth) and clear width is 33 − 3 = 30 in.
8 ft loft stair squeezed into a 48 in diameter
11 risers of 8.73 in are fine, but a 48 in stair leaves only 21.75 in of clear width, and 22.5° treads are just 5.6 in deep at the walkline — both below IRC minimums, so this layout would not pass as a required stair.
Frequently asked questions
What is the minimum size for a spiral staircase?+
Under the IRC, the clear width at and below the handrail must be at least 26 inches, which in practice means a stair diameter of about 60 inches or more once the center column is included. Smaller kits exist for secondary access where code allows.
What is the maximum riser height for spiral stairs?+
The 2021 IRC allows a rise of up to 9½ inches for spiral stairs, compared with 7¾ inches for ordinary residential stairs. All treads must be identical.
How much headroom does a spiral staircase need?+
The IRC requires at least 6 feet 6 inches (78 in) of headroom on a spiral stair, measured vertically from the leading edge of the tread to the structure above, including the tread one full turn up.
How many treads are in a full turn of a spiral stair?+
It depends on the tread angle: 30° treads give 12 per turn, 27.7° gives 13 and 22.5° gives 16. A typical 9 ft floor height with 30° treads uses 11 treads and turns about 330°.
What is the walkline on a spiral staircase?+
The walkline is the assumed path of travel, located 12 inches from the narrow end of the tread. The IRC requires each spiral tread to be at least 6¾ inches deep along that line, and the walkline radius may not exceed 24½ inches.
Results are general estimates and not legal advice. Laws vary by jurisdiction — consult a qualified attorney.