Quick answer: To size a spiral staircase, divide the total rise by a riser up to 9.5 inches for the number of risers, subtract one for treads, then divide your total rotation (usually 270-360°) by the treads for the degrees each tread turns.
This spiral stair calculator finds the number of treads, exact riser height, degrees per tread, total rotation, walkline tread depth, and floor opening size for a spiral staircase around a center pole.
A spiral staircase wraps identical pie-shaped treads around a central pole. Three numbers define it: how tall each step is (riser), how far each tread turns (degrees), and the diameter. Because spirals are compact, code allows them to be a little steeper than straight stairs.
Cut the finished floor opening about 2 inches larger than the outer diameter so the handrail and tread edges clear the framing. For a 60-inch spiral, frame a 62-inch (or slightly larger) rough opening, then trim to finish. Confirm the landing at the top gives a full-size tread of clearance before the first step down.
Divide the total floor-to-floor height by a target riser (spiral stairs allow up to 9.5 inches) to get the number of risers, then divide by that count for the exact riser height. The number of treads is one less than the risers, and each tread turns by the total rotation divided by the number of treads.
Most residential spirals rotate 270 to 360 degrees over a full flight. More treads spread over more rotation gives a gentler, more comfortable climb; fewer treads over less rotation feels tight and steep.
The IRC allows spiral stairs a steeper riser (up to 9.5 inches) but requires the tread depth at the 12-inch walkline to be at least 6.75 inches, a minimum clear width of 26 inches, and at least 6 feet 6 inches of headroom.
The finished floor opening should be about 2 inches larger than the outer diameter of the stair to leave clearance for the handrail and treads. A 60-inch spiral typically needs a 62-inch opening.
It depends on the height. For a 108-inch rise at an 8.5-inch riser you get about 13 risers and 12 treads. Taller ceilings simply add treads around the pole.