Spiral Staircase Calculator

Inputs

Units

Floor-to-floor height (mm)

Typical residential: 2600–3200 mm

Preferred riser (mm)

Min riser (mm)

Max riser (mm)

Outer diameter (mm)

Center column / inner opening diameter (mm)

Total rotation (degrees)

Common choices: 270, 360, 450

Walking-line factor (0–1)

2/3 of clear width is a common convention

Minimum going at walking line (mm)

Minimum inner going (mm)

Typical narrow-edge minimums are around 100–125 mm (≈4–5 in). Adjust to match your guideline.

Maximum outer going (mm)

Some standards also cap the outer going. Set your preferred limit here.

Tread thickness (mm)

Required headroom (mm)

Entry angle (deg)

Click the plan to set this quickly.

Landing offset (deg)

Exit radial = entry + rotation + offset

Results

Risers / Steps: —

Riser height: — mm

Deg per step (°): —

Steps per 360°: —

Clear width: — mm

Walking-line radius: — mm

Going at WL: — mm

Going inner/outer: — mm

Vertical rise per 360°: — mm

Helical length at WL: — mm

Exit radial angle: — °

Riser within range:

Checks against min/max riser inputs

Going at walking line:

Compares calculated going to minimum

Inner going limit:

g_i ≥ min inner going

Outer going limit:

g_o ≤ max outer going

Outer diameter

Inner/column

Walking line

One tread wedge

Entry/Exit radials

Example scenarios

Scenario	Floor-to-floor	Rotation	Outer dia	Column dia	Preferred riser	Min WL going	Tread thk	Req headroom
Residential Standard	2900	360°	1800	150	175	220	50	2000
Loft Tight Space	2800	270°	1500	120	180	220	45	2000
Commercial Roomy	3300	450°	2200	200	170	250	60	2100

Click “Load” to populate inputs. If you are in Imperial units, values convert to inches automatically. Fractional inputs like 6 1/2 or 6-3/8 are accepted.

Formulas used

Number of risers (steps): N = round( H / h_pref ), where H is total rise. Actual riser: h = H / N.
Degrees per step: θ = Θ / N, where Θ is total rotation in degrees.
Radii: inner r_i = D_i/2, outer r_o = D_o/2, clear width w = r_o − r_i.
Walking-line radius: r_w = r_i + f · w where f is the walking-line factor (default 2/3).
Going (tread depth along arc) at any radius r: g(r) = (2π r · θ) / 360. At inner, walking line, outer radii: g_i, g_w, g_o.
Steps per full turn: SPR = 360 / θ. Vertical rise per 360°: V₃₆₀ = h · SPR.
Headroom check (per 360°): V₃₆₀ − t ≥ R_head, where t is tread thickness and R_head is required headroom.
Helical length at walking line (approx): L ≈ N · g_w for total rotation Θ.

Local codes vary for minimum going at the walking line, maximum riser height, minimum clear width, guard/handrail requirements, and headroom. Use this tool for preliminary layout and verify with official standards.

How to use this calculator

Select your unit system. In Imperial mode you can enter fractional inches like 8 1/4, 7-5/8, or 10 3/16.
Enter floor-to-floor height, outer/inner diameters, and your riser preferences.
Set total rotation and the walking-line factor. If desired, set inner and outer going limits for additional checks.
Adjust entry angle and landing offset to visualize where the stair meets landings or openings. Click the plan to set entry angle.
Press Compute. Review the pass/fail badges for riser, goings at inner/walking line/outer, and headroom.
Export results to CSV/PDF and export the plan as SVG/PNG for coordination.

Inputs

Results

Example scenarios

Formulas used

How to use this calculator

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