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Spiral Staircase Calculator

Use this spiral staircase calculator to find the dimensions of a spiral staircase and the measurements of its treads for your DIY construction needs.

Clear
Treads1213 risers of 8.308 in, the top floor being the last step
Riser height8.3077 inspiral stairs are allowed up to 9½ in in most codes
Rotation per tread30°360° over 12 treads
Tread depth at the walkline9.425 inmeasured 12 in in from the outside, where codes want at least 7½ in
Walkline checkwithin codereduce the rotation or widen the stair if it fails
Stair diameter60 inradius 30 in
Tread outer arc length15.708 in
Clear walking width28 infrom the centre column to the outside
Headroom needed80 incodes want 6 ft 6 in on a spiral, measured at the walkline
Risers per full turn12
Floor opening needed62 inthe diameter plus about an inch each side

The method

treads = rise ÷ riser; each tread turns 360° ÷ treads per turn

The walkline is what codes measure

A spiral tread is a wedge, so its depth depends where you stand. Codes measure 12 inches in from the outer edge — the walkline — and generally require at least 7½ inches of tread there, with risers up to 9½ inches allowed because the geometry makes anything gentler enormous.

Rotation and comfort trade against each other directly. Fewer treads per turn means each one sweeps further and is deeper at the walkline, but the stair gets steeper. More treads per turn gives a gentler climb and narrower wedges.

Remember the floor opening has to clear the outside diameter with room for the handrail, and that furniture does not go up a spiral stair. That is the usual reason people regret one.