Plan a spiral staircase in seconds. Enter the floor-to-floor height, the stair diameter and how far it turns to get the number of risers and treads, riser height, tread angle, tread depth at the walkline, clear width and headroom, checked against the IRC spiral stair rules.
Last updated:
Units
Finished floor below to finished floor above (108 in = 9 ft)
Outside diameter, e.g. 60 in = 5 ft
°
IRC spiral limit 9.5 in (24.1 cm)
Space the outer rail takes from the walking width
Enter your values to see the result.
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How to use the Spiral Staircase Calculator
Measure floor to floor, from finished floor below to finished floor above.
Enter the stair diameter; 5 ft (60 in) is the smallest size that usually meets the 26 in width rule.
Choose the rotation that fits your entry and exit directions.
Adjust the column, tread thickness and rail allowance to match your kit.
Press Calculate and check the four code results and the plan view.
Spiral staircase formulas
Risers n = ⌈floor height ÷ max riser⌉; riser height = floor height ÷ n
Handrail length = √((2π r × rotation/360)² + height²)
Worked example: 108 in floor height, 60 in diameter, 360° rotation, 4 1/2 in column. 12 risers of 9 in, 11 treads, θ = 30°. Walkline radius = 2.25 + 12 = 14.25 in, so tread depth = 2π × 14.25 × 30 ÷ 360 = 7.46 in (passes 6 3/4 in). Clear width = 30 − 2.25 − 1.5 = 26.25 in (passes 26 in). Headroom = 12 × 9 − 1.5 = 106.5 in.
Typical spiral stair sizes
Diameter
Clear width*
Walkline depth at 30°
Typical use
4 ft (48 in)
20 1/4 in
7 7/16 in
Lofts, secondary access (often below code width)
4 ft 6 in (54 in)
23 1/4 in
7 7/16 in
Tight spaces where allowed
5 ft (60 in)
26 1/4 in
7 7/16 in
Smallest common code-compliant home size
5 ft 6 in (66 in)
29 1/4 in
7 7/16 in
Comfortable main stair
6 ft (72 in)
32 1/4 in
7 7/16 in
Easy to carry items
*With a 4 1/2 in column and 1 1/2 in handrail allowance. The walkline depth depends on the column, not the diameter; bigger stairs gain width rather than walkline depth.
Tips for planning a spiral stair
Decide where you step on and off first: the rotation must line up the bottom entry and the top landing with your doorways and hallways.
Remember the landing sits at floor level and is usually larger than a tread; leave room for it in the opening.
Furniture rarely fits up a spiral stair; plan another route for large items.
Check local codes: some areas restrict spiral stairs as the only exit from a sleeping area.
Frequently asked questions
How do I calculate the number of treads on a spiral staircase?
Divide the floor-to-floor height by the maximum riser height and round up to get the number of risers. The number of treads is one less, because the top landing serves as the last step. For a 108 in floor height and a 9 1/2 in maximum riser, 108 ÷ 9.5 = 11.4, so 12 risers of 9 in and 11 treads plus the landing.
What is the tread angle of a spiral staircase?
Tread angle = total rotation ÷ number of risers. A 360° stair with 12 risers has 30° treads, while the same stair turning 330° has 27.5° treads. Smaller angles make shallower treads, so a larger diameter or more rotation helps.
What are the code requirements for spiral stairs?
The International Residential Code (R311.7.10.1) requires a clear width of at least 26 in at and below the handrail, a tread depth of at least 6 3/4 in measured at the walkline 12 in from the narrow edge, identical treads, a riser height of no more than 9 1/2 in, and headroom of at least 6 ft 6 in. Commercial buildings follow the IBC, which only allows spiral stairs in limited situations.
How much headroom does a spiral staircase need?
At least 6 ft 6 in (78 in) under the residential code. On a spiral, the step one full turn above you sets the headroom, so headroom = steps per 360° × riser height − tread thickness. With 30° treads and 9 in risers, 12 × 9 − 1.5 = 106.5 in.
What diameter spiral staircase do I need?
To meet the 26 in clear width, most residential spiral stairs need a diameter of at least 60 in (5 ft). Smaller 4 ft to 4 1/2 ft stairs are common for lofts and secondary access where local code allows. Wider stairs, 6 ft and up, are much easier for carrying things.
How big should the floor opening be?
Kit makers usually recommend a square or round opening about 2 in larger than the stair diameter, so a 60 in stair needs roughly a 62 in opening. Always check the manufacturer's drawing, because landing shape and rail details vary.
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