Spiral stairs fit into a small footprint but must still meet code. The International Residential Code (R311.7.10.1) requires a clear width of at least 26 in, risers no taller than 9½ in, treads at least 6¾ in deep measured 12 in from the narrow edge, and 78 in of headroom. The total rotation – often 270° or 360° – is chosen so the top landing faces the right way.
How to use the Spiral Staircase Calculator
- Total rise (floor to floor)
- Maximum riser height
- Stair diameter
- Centre pole diameter
- Total rotation
- Tread thickness
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the floor-to-floor rise, the maximum riser height, the stair diameter, the centre pole diameter, the total rotation and the tread thickness. The calculator finds the number of risers, the exact riser height, the angle of each tread, the walkline tread depth, the headroom and the handrail length.
IRC spiral stair minimums
| Item | Requirement |
|---|---|
| Clear width | 26 in (660 mm) |
| Riser height | ≤ 9½ in (241 mm) |
| Tread depth at walkline | ≥ 6¾ in (171 mm) |
| Headroom | ≥ 78 in (1,982 mm) |
Tips for an accurate estimate
- Sketch the job and split irregular areas into rectangles, circles and triangles before you measure.
- Add a waste allowance – typically 5–10% for simple layouts and 15–20% for diagonal, curved or heavily cut work.
- Check the coverage or yield printed on the product, because sizes and densities vary between manufacturers.
- Buy all units of a finished material (tile, brick, siding) from the same batch so colours match, and keep a few spares for repairs.
- Confirm local prices – quarry, lumber and steel prices vary widely by region and season.
Common mistakes to avoid
- Ordering exactly the calculated amount with no allowance for breakage, offcuts and settlement.
- Forgetting to subtract doors, windows and other openings – or subtracting small ones that still need cutting around.
- Confusing US short tons (2,000 lb) with metric tonnes (1,000 kg) when comparing quotes.
Inputs explained
| Field | What to enter |
|---|---|
| Total rise (floor to floor) | Units: in, cm, mm, ft, m |
| Maximum riser height | Units: in, cm, mm, ft, m |
| Stair diameter | Units: in, cm, mm, ft, m |
| Centre pole diameter | Units: in, cm, mm, ft, m |
| Total rotation | Units: °, rad |
| Tread thickness | Units: in, cm, mm, ft, m |
What each result means
- Risers = ceil([Total rise (floor to floor)]÷[Maximum riser height] – 0.000001)
- Treads (excluding top landing) = [Risers] – 1
- Riser height = [Total rise (floor to floor)]÷[Risers]
- Angle per tread = [Total rotation]÷[Risers]
- Tread depth at walkline = min([Centre pole diameter]÷2 + 0.3048, [Stair diameter]÷2)×rad([Angle per tread])
- Headroom per full turn = 360÷[Angle per tread]×[Riser height] – [Tread thickness]
- Clear tread width = [Stair diameter]÷2 – [Centre pole diameter]÷2
- Handrail length (outer) = sqrt((pi×[Stair diameter]×[Total rotation]÷360)^2 + [Total rise (floor to floor)]^2)
Formula
Risers = ceil(total rise ÷ maximum riser)Tread angle = rotation ÷ risers (landing counts as the last tread)Walkline depth = (pole radius + 12 in) × angle (radians)Headroom ≈ rise per 360° − tread thicknessWorked example
Inputs — Total rise (floor to floor): 108 in; Maximum riser height: 9.5 in; Stair diameter: 60 in; Centre pole diameter: 4 in; Total rotation: 360 °; Tread thickness: 1.5 in.
Results — Risers = 12; Treads (excluding top landing) = 11; Riser height = 9 in; Angle per tread = 30 °; Tread depth at walkline = 7.33 in; Headroom per full turn = 106.5 in; Clear tread width = 28 in; Handrail length (outer) = 18.1 ft.
Key terms
- Walkline
- The path 12 in out from the narrow end of the treads, where tread depth is measured.
Frequently asked questions
What is the minimum diameter for a spiral staircase?
To get the 26 in clear width required by the IRC you need about a 60 in diameter with a 4–5 in pole.
How many treads per 360°?
Commonly 12 treads plus the landing in a full turn; fewer treads means taller risers or more rotation.
What is the headroom rule?
78 in minimum (IRC); with 9 in risers and 12 treads per turn you get about 106 in less the tread thickness.
What formula does the Spiral Staircase Calculator use?
Risers = ceil(total rise ÷ maximum riser).