A spiral stair's outer diameter has to fit the floor opening it rises through, with clearance for the handrail. Residential spirals commonly run 48 to 60 inches in diameter. Larger diameters give deeper, easier treads but consume more floor.
Clear width is the outer radius minus the center column radius. The IRC requires at least 26.0 inches of clear width at and below the handrail. A fat structural column eats into that, so size it as slim as the structure allows.
Spiral stairs are allowed steeper risers than straight stairs — up to 9.5 inches under the IRC — because they are typically secondary access. Tread depth is measured along a walking line near the outer edge, and the IRC requires at least 6.75 inches there.
The total rotation (often 360 degrees over the full rise) divided by the tread count sets how far each tread sweeps. Headroom is governed by the rise accumulated in one full turn: a person must clear the underside of the flight above, at least 78.0 inches under the IRC.
The spiral calculator computes arc tread depths, clear width, rise per turn and handrail length as you adjust each input.
Suppose a 108-inch total rise through a 60-inch-diameter opening with a 4-inch center column. Clear width is the outer radius (30) minus the column radius (2) = 28 inches, clearing the IRC's 26-inch minimum. At a 9-inch target riser, 108 / 9 = 12 risers. Over a 360-degree turn, each tread sweeps 360 / 12 = 30 degrees. Headroom is governed by the rise in one full turn: 12 risers × 9 inches = 108 inches per turn, comfortably above the 78-inch spiral minimum for anyone passing beneath.
The IRC permits taller risers (up to 9.5 inches) and shallower walking-line treads (6.75 inches minimum) on spirals than on straight stairs, because spirals are treated as secondary or space-saving access rather than a primary egress route. That allowance is what lets a spiral fit a full storey into a circle a few feet across — but it also means spirals climb steeply and aren't ideal as the only stair in a home, or for moving furniture.
A larger diameter gives deeper treads at the walking line and an easier climb, but consumes more floor and a bigger ceiling opening. A 48-inch spiral is the tight minimum for occasional use; 60 inches is far more comfortable for daily use. Always size the diameter to the opening first, then confirm the clear width still clears 26 inches after subtracting the column.
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