Spiral Staircase Diameter Calculator — BS 5395-2 Categories & Geometry
Spiral staircases follow their own standard — BS 5395-2 — which classifies every spiral by who uses it (Category A secondary domestic, B primary domestic, C semi-public/commercial) and sets minimum clear widths and walking-line goings for each. This calculator takes your diameter, floor height and category, and checks the three things that decide whether a spiral works: clear tread width, going on the walking line, and the one everyone forgets — headroom per revolution, because on a spiral you climb under yourself. When it fails, it tells you the smallest diameter that passes.
Spiral Diameter Calculator
Diameter + height + category — width, going and headroom checked in one pass.
Minimum spiral staircase sizes in the UK come from BS 5395-2, by category: Category A (secondary domestic — loft rooms, occasional access) needs a clear tread width of 600mm and a walking-line going of 145mm, workable from about 1,300mm diameter; Category B (the main staircase of a home) needs 800mm width and 190mm going — realistically 1,600mm diameter and up; Category C (offices, semi-public) needs 800mm and 230mm, pushing diameters toward 1,800–2,200mm. The hidden constraint is headroom per revolution: you pass beneath your own treads, so one full turn must climb at least ~2,000mm plus structure — which caps how gentle the rise can be on small diameters.
BS 5395-2 Categories — Reference Table
| Category | Use | Min Clear Width | Min Going @ Walking Line | Typical Diameter |
|---|---|---|---|---|
| A — Secondary | One room, loft, occasional | 600mm | 145mm | 1,300–1,500mm |
| B — Primary domestic | Main stair of a dwelling | 800mm | 190mm | 1,600–1,800mm |
| C — Semi-public | Offices, small commercial | 800mm | 230mm | 1,800–2,200mm |
| D / E — Public & assembly | Shops, venues, crowds | 900–1,000mm | 250mm | Engineer designed |
The three checks in plan: clear width from the column, going on the walking line, and headroom gained per revolution.
The "it's just for the loft" trapCategory A exists for genuinely secondary access — one room, occasional use. The moment a spiral is the only way to reach a habitable floor used daily, it's Category B, and a 1,300mm Category A spiral bought online won't pass Building Control for that job. It's the spiral equivalent of the mezzanine "staff only ≠ utility" mistake: classify by real use, then choose the diameter. Not sure a spiral fits at all? Compare footprints with the rise & going calculator and the winder calculator — an L-shape with winders often beats a spiral in the same corner.
The Headroom-Per-Revolution Rule — The Constraint Nobody Sees Coming
On a straight flight, headroom is about the ceiling. On a spiral, after roughly three-quarters of a turn you're walking under your own staircase — so the height climbed in one full revolution, minus the structural depth of the treads above, must leave 2,000mm of clear headroom. With 12 treads per revolution at a 193mm rise, one turn climbs 2,316mm: subtract ~150mm of tread structure and you clear the limit — barely. Drop to a gentler 170mm rise and the same spiral fails, which is the counterintuitive result: on spirals, a steeper rise can be the compliant choice. Fewer treads per revolution (larger rotation per tread) also helps headroom, but costs going on the walking line — the calculator balances all three so you can see the trade instantly.
Spirals as fire escapes — mostly no. Spiral and helical stairs are generally not acceptable on escape routes serving members of the public, and are restricted even for staff escape — BS 5395-2 and Approved Document B both limit their use where an evacuation flow is expected. If the staircase has any escape function, size it as a conventional flight with the escape stair width calculator before falling in love with the spiral.
Spiral Staircases — FAQ
Common questions about spiral staircase sizes and UK rules.
The Right Category, The Right Diameter
Steel spiral and helical staircases designed to BS 5395-2 — category classified, walking-line geometry verified in CAD, guarding to BS 6180, EN 1090 UKCA documentation included. Free survey, fixed price within 24 hours.