Spiral stair calculator

A spiral stair climbs a set floor-to-floor rise while turning through a set rotation, commonly a full 360 degrees for one complete turn, though tighter spaces sometimes use less rotation across a shorter rise. Diameter sets how far each tread extends from the center column.

Riser count is calculated the same way as any stair: divide total rise by your target riser height and round to the nearest whole number. For a 108 in rise at an 8.5 in target riser, that's 108 ÷ 8.5 = 12.7, rounded to 13 risers, giving an actual riser height of 108 ÷ 13 ≈ 8.31 in.

Spiral stairs commonly run slightly steeper risers than straight stairs, since a tighter footprint usually means trading some comfort for a smaller diameter. Confirm any riser height maximum with your local code before finalizing.

With 13 risers spread across a full 360-degree turn, each tread occupies 360 ÷ 13 ≈ 27.7 degrees of rotation. That angle-per-tread number is what defines how wide each wedge-shaped tread is at any given radius from the center.

If the stair only turns, say, 270 degrees over the same rise, each tread would occupy 270 ÷ 13 ≈ 20.8 degrees instead, a narrower wedge at every point, which usually means a wider diameter is needed to keep the walk-line tread depth adequate.

Like winder treads, spiral treads taper from wide at the outside edge to a point at the center column, so tread depth is measured along a walk line, commonly cited as 12 in in from the narrow edge closest to the center. For a 60 in diameter stair, that walk line sits at a radius of 60 ÷ 2 − 12 = 18 in.

Tread depth along that walk line is the arc length there: 2π × 18 × (27.7 ÷ 360) ≈ 8.7 in for the 13-riser, 360-degree example above. That's below the commonly cited 10 in minimum used for straight treads, but spiral stairs are generally governed by separate, spiral-specific reference values that commonly cite a smaller minimum, often around 7½ in, in exchange for a taller allowed riser.

Because spiral stairs run under their own code section in many jurisdictions rather than the general straight-stair rules, always confirm the applicable spiral-specific minimums locally rather than applying straight-stair numbers directly.

Headroom on a spiral stair has to be checked all the way around the turn, not just at one point, because the tread directly below any given point rotates to a different position than it would in a straight flight. A low ceiling or overhead beam that clears one side of the spiral may not clear the opposite side.

Commonly cited residential reference values still call for roughly 6 ft 8 in of headroom measured vertically above the nosing line at every point around the turn, hedged since exact requirements and any spiral-specific allowances vary by jurisdiction.

Diameters around 60 in are commonly cited as a practical minimum for comfortable primary-use spiral stairs, with some jurisdictions allowing smaller diameters, sometimes down to around 48 in, for space-saving or secondary-access spiral stairs under specific reference values. Larger diameters, 66 in and up, give noticeably deeper walk-line treads and easier hand-to-rail clearance.

As the table above shows, walk-line tread depth improves quickly with diameter, since arc length scales with radius. Going from 48 in to 60 in nearly gains 3 in of walk-line depth without changing the riser count at all.

Most residential spiral stairs installed today are prefabricated kits sized to a handful of standard diameters, commonly in the 48 in to 72 in range, with fixed riser heights that you select to match your total floor-to-floor rise. Kits are generally faster to install and pre-engineered to meet the reference values they're sold under, but they lock you into their fixed diameter and rotation options.

A custom spiral stair, built or fabricated to exact dimensions, is worth considering when the available rise or diameter falls between standard kit sizes, or when the design calls for a nonstandard rotation like a 270-degree turn instead of a full 360.