One of the most basic calculations you'll need to make for a spiral staircase is the rise, or height. This applies not only to the staircase as a whole, but also to the height of each step. The total rise will depend on the height of your room; a 10-foot room will require a staircase with a rise of 120 inches, plus the thickness of the ceiling and the upstairs floor, in order for the highest step to be level with the upper floor.
A spiral staircase uses treads, which are comparable to the individual stairs in a conventional staircase. Treads can be attached at their ends by a spiraling support, or they can attach to a central post that is taller than the staircase itself. In either case, the number of treads depends on the rise between each tread and the total rise of the spiral staircase. You can usually subtract one from the total number of treads, as there's no need for a tread at the bottom of the staircase where it begins, level with the lower floor of your home.
A spiral staircase can have any rotation, which is the amount of turn between the lowest and highest treads. Some spiral staircases have a 360-degree rotation, meaning that you end up at the top, facing the same direction you started out facing at the bottom. A 180-degree spiral staircase has half as much rotation, while a 540-degree spiral includes a full turn, plus half of an additional turn. The location of the staircase and the presence of any doors or barriers will determine the rotation you need, which, in turn, impacts the width and position of treads.
Weight is an important safety consideration for spiral stairs. If the stairs use a central post for support, you'll need to determine its weight rating and subtract the weight of the treads and handrails to calculate the maximum safe weight for people -- and items they carry -- on the stairs at a given time. If your spiral stairs mount to the floor joists on an upper floor, their strength will be a factor. Prefabricated spiral staircases come with weight ratings, which should serve as a guide for safe use.