Spiral Staircase Calculator
Design a spiral staircase: number of risers and treads, riser height, angle per tread, tread depth at the walkline, clear width, headroom and floor opening, checked against the IRC spiral stair rules.
How it works
risers n = ⌈height ÷ max riser⌉ · riser = height ÷ n · angle per tread = rotation ÷ n · tread at walkline = 2π × (column radius + 30.5 cm) × angle ÷ 360 · clear width = radius − column radius · headroom ≈ (360 ÷ angle) × riser − 5 cm tread
A spiral stair winds its treads round a centre column. The floor-to-floor height is divided into equal risers no taller than the chosen maximum, and the total rotation (the top landing counts as the last step) is shared between them. The International Residential Code (R311.7.10.1) measures the tread 12 in (30.5 cm) out from the narrow end — the walkline — and requires at least 6¾ in (17.1 cm) there, a clear width of 26 in (66 cm), risers of no more than 9½ in (24.1 cm) and 78 in (198 cm) of headroom. Headroom is roughly one full turn of risers minus the tread thickness, so tighter rotations per step raise it. Leave a floor opening about 5 cm wider than the stair.
Worked example
A 280 cm floor-to-floor height, 160 cm diameter, one full turn and a 22 cm maximum riser: n = ⌈280 ÷ 22⌉ = 13 risers of 21.54 cm and 12 treads plus the landing, at 360 ÷ 13 = 27.69° each. With a 10.2 cm column the walkline radius is 35.58 cm, so the tread there is 2π × 35.58 × 27.69 ÷ 360 = 17.20 cm — just over the 17.1 cm minimum. Clear width = 80 − 5.1 = 74.9 cm and headroom ≈ 13 × 21.54 − 5 = 275 cm.
Sources: International Residential Code R311.7.10.1 — Spiral stairways (ICC) · Stairway Manufacturers’ Association — Visual interpretation of the IRC stair provisions
Frequently asked questions
What diameter spiral staircase do I need?
To meet the IRC 26″ clear width with a 4″ column you need at least about 56″ (142 cm) of diameter, and about 60″ (152 cm) once the handrail is allowed for. Smaller 4 ft (122 cm) stairs only suit secondary access where codes allow.
How many steps in a spiral staircase?
Divide the floor-to-floor height by the riser height: about 12–14 risers for a 2.6–3.0 m storey. One of the divisions is the top landing, so there is one fewer tread than risers.
What is the walkline of a spiral stair?
The line people naturally walk on, taken by the IRC as 12 in (30.5 cm) from the narrow inner end of the treads. Tread depth is measured there and must be at least 6¾ in.
How much headroom does a spiral staircase need?
The IRC asks for 78 in (198 cm) for spiral stairs (80 in for straight stairs). Because the stair passes under itself after one turn, a full turn of risers must add up to roughly 2.03 m or more.
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This calculator is for educational purposes. Double-check important results before acting on them.