Spiral Staircase Calculator
Tread count, angle and dimensions for a spiral staircase.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Risers = ⌈ Total rise ÷ target riser ⌉; Riser height = Total rise ÷ risers
Rotation per tread = Total rotation ÷ (risers − 1)
Walkline tread depth = (rotation per tread in radians) × (outer radius − 12 in)
Handrail length = √( (total rotation in radians × handrail radius)² + Total rise² )
Run the calculation to see each step with your own numbers.
9 ft rise, 60 in outer diameter, 270° rotation, 9 in target riser, 12 in walkline.
Risers = ⌈108 ÷ 9⌉ = 12; riser height = 9.0 in
Rotation per tread = 270° ÷ 11 = 24.5°
Walkline depth = 0.428 rad × 18 in = 7.7 in ✓ over 6¾ in
Handrail ≈ √((4.712 × 28.5 in)² + 108²) ≈ 14.9 ft
- A true helix with equal risers and equal rotation per tread around a central column.
- Tread depth is measured at the 12-in walkline per the IRC — the wide (outer) end is deeper, the narrow end shallower.
- Code minimums shown (9½ in riser, 6¾ in walkline tread, 6 ft 6 in headroom, clear width) are from the IRC for reference — spiral stairs are permitted only in limited locations; check your code.
- The handrail length is the helical arc along the outside; add for returns and overhangs.
- Why is a spiral tread depth so small?
- Because it's measured near the centre where the tread is narrow. Every spiral tread is a wedge — wide at the outside, narrow at the column. The IRC fixes the measuring point at 12 in from the narrow edge.
- How much rotation should the stair turn?
- About 270 degrees is common for one storey in a typical kit diameter — enough that each wedge-shaped tread is a usable depth without the stair feeling cramped. A larger diameter can turn less; a tight column needs more.
- What diameter do I need?
- Kits run from about 42 to 72 inches overall. Bigger is easier to climb and to carry furniture up, but takes more floor. The walkline tread depth the calculator reports tells you whether a given diameter and rotation still work.
Geometry
Standard helix geometry (arc length = angle × radius; helical arc = √((θr)² + rise²)).
Code limits (reference)
IRC R311.7.10.1 Spiral stairways. Shown for reference only; BuildFactor does not certify code compliance.
About the Spiral Staircase Calculator
The Spiral Staircase Calculator works out the geometry of a helical stair — the riser count and height, the rotation per tread, the tread depth measured at the code walkline 12 inches in from the narrow edge, and the length of the helical handrail — from the total rise, diameter and total rotation. Spiral stairs are steeper than straight stairs and are only permitted in limited locations, so check where your code allows one.
What you enter
- Total rise — Finished floor to finished floor.
- Outer diameter — The overall diameter the stair occupies. Common kits are 42–72 in.
- Centre column diameter
- Total rotation (degrees) — How far the stair turns from bottom to top. 270° is common for one storey.
- Target riser height — Spiral stairs are steeper — IRC allows up to 9½ in.
- Walkline offset from outer edge — IRC measures tread depth 12 in in from the narrow edge.
What you get back
- Number of risers — the exact result of the calculation.
- Riser height — IRC R311.7.10.1: 9½ in maximum for a spiral stair.
- Rotation per tread — the exact result of the calculation.
- Walkline tread depth — IRC minimum 6¾ in at the 12-in walkline.
- Clear height at start needed — IRC R311.7.10.1: 6 ft 6 in minimum spiral headroom — check your opening.
- Handrail length — the exact result of the calculation.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Stair Calculator — Stair rise, run, stringer length and step count for a total height.
- Spindle Spacing Calculator — Evenly spaced baluster / spindle layout for a railing run.
- Concrete Stairs Calculator — Concrete volume for a poured stair flight.
Part of the Stairs calculators.