Designers determine the spiral stair diameter by considering various factors. For instance, will you climb the spiral staircase often, or not so much. Will the stair access a large area above, or a small area. What kind of spiral stair do you want, wood, or metal. Will you have to meet building codes . And of course, how much do you plan to spend.
You can navigate a code approved spiral stair more comfortably than a smaller size. Wider spiral stair diameters increase the clear walking space. This is the area between the center column and the spiral handrail.
Code-approved spiral stair diameters
Codes require a tread width of at least 26″. A spiral stair diameter of about 60″ or larger will provide this width depending on whether the stair is wood or steel. (A steel spiral can provide 26″ with a slightly smaller diameter because it has a smaller center column — say a 3″ or 4″ steel tube, compared to a 6″ or more wood column.)
We would not recommend a small spiral stair diameter for a main staircase. You might make an exception a very small home with limited space. But you would be sacrificing comfort and building codes may not allow the smaller stair.
A smaller-than-code spiral stair diameter can work if you expect light traffic, say to a private loft space or to an attic. And building inspectors sometimes may allow such use.
A narrow spiral might also be allowed if there is another way to escape in case of fire. For instance, you could get out via a second, code-approved stairway. Or in the case of a spiral to a walk-out basement, you could use the exterior basement doors to escape. But only the local inspector can make these decisions. Check with your building department if you are planning a small spiral.
More advantages of the spiral stair diameter, large or small
Wider spiral stairs (say 72″ and up) can be designed to have more treads that are narrower, so that you turn less per step. This has two possible advantages. One, adding more treads could lower the rises between each tread. Or two, the same number of narrow treads will turn less up the entire spiral and this could improve your access at the top or bottom floor.
Narrower diameter spiral stairs are tight to climb. A 48″-diameter stair has a step width of barely 18″. Larger persons may need to turn their shoulders as they go up. However, tight stairs may seem safer to some. Because you climb between the center column and the spiral rail curving close around you. So you are effectively closed in and it would be difficult to fall.
Spiral stair diameters may be measured differently by different manufacturers. One company may measure to the outside of the handrail, or to the outside of balusters beyond the handrail. Another may measure 1-1/2″ outside the handrail for finger room. Their 60″ (nominal) diameter would measure 57″ to the outside of the rails.