How to use the spiral staircase calculator
Spiral stairs save floor area, but they are tightly controlled by code because every dimension interacts: the diameter sets the width and tread depth, the number of treads per turn sets the headroom, and the floor height sets the riser. This calculator takes the few numbers you know early on and works out the rest, then flags anything that falls short of the US residential and building code rules for spiral stairways.
- Choose units. US mode works in inches; metric mode works in millimetres with the equivalent limits.
- Enter the floor-to-floor height, finished floor to finished floor, not the ceiling height.
- Enter the outside diameter of the stair and the centre column diameter from the kit or the structural design.
- Choose treads per full turn. This sets the angle of each tread.
- Enter the tread thickness and handrail allowance, the space the handrail takes from the clear width.
Spiral stair formulas
Risers = ⌈floor-to-floor ÷ 9½ in⌉ · riser height = floor-to-floor ÷ risers
Treads = risers − 1 (the top floor or landing is the last step)
Tread angle = 360° ÷ treads per turn · rotation to landing = treads × tread angle
Walkline radius = column radius + 12 in (300 mm)
Tread depth at walkline = 2π × walkline radius × tread angle ÷ 360°
Clear width = outside radius − column radius − handrail allowance
Headroom = treads per turn × riser − tread thickness
| Rule (IRC R311.7.10.1 / IBC 1011.10) | US | Metric equivalent |
|---|---|---|
| Minimum clear width at and below handrail | 26 in | 660 mm |
| Minimum tread depth, 12 in from narrow edge | 6¾ in | 171 mm |
| Maximum riser height | 9½ in | 241 mm |
| Minimum headroom | 6 ft 6 in (78 in) | 1,981 mm |
All treads must be identical, and the rotation of the stair must be the same direction all the way up. Add a top landing, often a quarter turn, to the total rotation when planning where people step off.
Worked examples
A 5 ft spiral stair to a loft
A loft sits 108 inches above the floor below. With a 60 inch diameter, a 4½ inch column and 12 treads per turn, the stair needs ⌈108 ÷ 9.5⌉ = 12 risers of 9 inches and 11 treads, turning 330° before the landing. The walkline sits 2.25 + 12 = 14.25 inches from the centre, so each 30° tread is 2π × 14.25 × 30 ÷ 360 = 7.46 inches deep there. The clear width is 30 − 2.25 − 1.5 = 26.25 inches, and headroom is 12 × 9 − 1.5 = 106.5 inches. Every rule passes, with the width just over the 26 inch minimum.
A metric stair
A 2,800 mm floor-to-floor stair with a 1,600 mm diameter and a 114 mm column needs ⌈2,800 ÷ 241⌉ = 12 risers of 233 mm. At 12 treads per turn, the walkline radius is 57 + 300 = 357 mm and the tread depth there is 187 mm. With a 40 mm handrail allowance, the clear width is 800 − 57 − 40 = 703 mm, and headroom is 12 × 233 − 40 = 2,760 mm.
Who this calculator helps
- Architects testing whether a spiral stair fits a loft, mezzanine or roof access before ordering a kit.
- Interior designers planning a feature stair in a compact apartment or shop.
- Builders and stair fabricators checking a client's dimensions against code before quoting.
- Homeowners comparing kit diameters for a basement or attic conversion.
Practical tips
- Plan the floor opening about 2 inches (50 mm) larger than the diameter, and check it with the stairwell opening calculator logic if the stair is not centred in the well.
- If the space allows a straight run, compare comfort and cost with the stair rise and run calculator.
- Spiral stairs are hard for moving furniture. Check walkways around the stair with the furniture clearance guide.
- Use the feet and inches converter when a kit is specified in metric and drawings are in feet.
Want to show the client how the stair will look in the room? Turn a sketch of it into a photoreal image with Sketch to Render.
