What building codes actually require: how wide is a spiral staircase?

22 July 2026

Before you decide on a spiral staircase – its diameter, its position in the room, how much floor space it will occupy – there is one conversation that needs to happen first: the one with the building code. Understanding how wide is a spiral staircase and what that width means in practice starts with the building code. Dimensions are not a matter of preference, they are regulated, and the regulations exist for good reasons: safety, usability, and the ability to move through a staircase comfortably over years of daily use.

This guide covers the minimum dimensional requirements for a residential spiral staircase under two reference frameworks: the International Residential Code (IRC), which applies across most of the United States, and the Italian standards (UNI 10804 and local building regulations), which govern residential construction in Italy. If you are planning a project in either country, or simply want to understand what the numbers mean before speaking with a designer, this is the place to start.

Spiral staircase dimensions under the US Building Code (IRC)

The IRC dedicates a specific section to spiral staircases (R311.7.10.1), recognizing them as a distinct staircase typology with their own dimensional rules, separate from straight-run or winder stairs.The key requirements for a residential spiral staircase are:

Minimum clear width at and below the handrail: 26 inches (660 mm). This is the usable walking width, the space between the center column and the handrail; it defines how comfortably a person can navigate the staircase.

Minimum tread depth at the walkline: 6¾ inches (171 mm). The walkline is measured 12 inches from the narrow end of the tread; this is the point where foot placement naturally occurs during a normal ascent or descent.

Maximum riser height: 9½ inches (241 mm). This is the vertical distance between one tread and the next, higher than what is permitted for standard residential stairs, reflecting the compact nature of the spiral staircase.

Minimum headroom: 6 feet 6 inches (1,981 mm). Measured vertically from any tread nosing to the ceiling or obstruction above.

Minimum overall diameter: 60 inches (approximately 152 cm). This is the minimum floor space needed for a spiral staircase to meet the clear width requirement at the walkline; the IRC does not vary this figure by material; specific manufacturers may apply different tolerances for metal versus wood construction, but the code minimum is a single threshold.

All treads must be identical, no variation in rise or depth is permitted within the same flight.

One important note: the IRC governs one- and two-family dwellings. Commercial buildings fall under the International Building Code (IBC), which has stricter requirements; local jurisdictions may also adopt amendments to the IRC, so always verify with your local building department before finalizing a design.

Spiral staircase dimensions under Italian standards

Italian regulations for spiral staircases draw from several overlapping frameworks: UNI 10804 (a technical standard for prefabricated helical staircases), the Testo Unico dell’Edilizia, and local building regulations. NTC 2018 governs structural safety rather than dimensional minimums for use, so the figures below reflect technical standards and municipal building codes rather than the NTC directly. The result is a system that distinguishes clearly between two contexts of use.

Primary private context: a staircase that serves as the main connection between habitable floors in a residence; minimum diameter: 160 cm, minimum usable passage width: 70 cm

Secondary private context: a staircase that supplements a primary staircase ( access to an attic, a mezzanine, a secondary level). Minimum diameter: 120 cm, minimum usable passage width: 50 cm

It is technically possible to install a spiral staircase with a diameter as small as 100 cm, but at that size the usable passage drops below 50 cm, a dimension that raises serious ergonomic and safety concerns for everyday use. This kind of installation may be tolerated as a secondary access in exceptional cases, but should never be the primary connection between floors.

The riser-to-tread ratio follows the standard ergonomic formula (2 × riser + tread = 63 cm), applied within the constraints of the chosen diameter; for diameters up to 160 cm, with 12 treads per revolution, the riser height typically falls between 20 and 24 cm. For larger diameters up to approximately 250 cm, with 16 treads per revolution, the riser height drops to between 18 and 22 cm, producing a more comfortable and natural stride. The minimum recommended tread depth at the walkline is 25 cm, a threshold that becomes particularly relevant during descent, where a shallower tread increases the risk of misstep.

Standard spiral staircase width: what “minimum” really Means

The question of how wide is a spiral staircase has a different answer depending on whether you are looking at the code minimum or at what actually works in daily use. The standard spiral staircase width of 26 inches (66 cm) under the IRC, or 70 cm under Italian standards for a primary staircase, represents the lower threshold of safe usability. At these dimensions, a spiral staircase is functional for a single person navigating it regularly, it will not accommodate two people passing simultaneously, and it will make moving furniture or large objects between floors extremely difficult.

The space needed for a spiral staircase that genuinely works well in a residence, as a daily-use element, not just a secondary access, is typically a diameter of 160–200 cm (approximately 63–79 inches). At this size, the tread depth at the walkline becomes genuinely comfortable, the riser height stays within an ergonomic range, and the staircase begins to feel less like a compromise and more like an architectural choice. This is the range within which most of Officine Sandrini’s residential spiral staircases are designed. The minimum tells you what is allowed. The project tells you what is right for that specific space.

How much space is needed for a spiral staircase?

The floor opening required for a spiral staircase corresponds to its outer diameter plus any structural tolerances required for installation. For a staircase with a 160 cm diameter, the floor opening will typically be 160–165 cm; for a 200 cm diameter staircase, the opening will be proportionally larger.

What often surprises people planning a residential spiral staircase size is how the vertical dimension interacts with the horizontal one: a smaller diameter staircase needs more revolutions to cover the same floor-to-floor height, which means more treads, a steeper pitch, and less headroom per revolution; a larger diameter allows fewer, shallower treads and a more generous headroom clearance.

The calculation is not just about the hole in the floor, it involves the full three-dimensional volume the staircase will occupy: the diameter at the base, the height of the ceiling opening, and the clearance required at every point along the ascent. Getting this right requires a proper survey of the space, measurements taken on site, not estimated from a floor plan.

If you are still asking how wide is a spiral staircase for your specific project, the only reliable answer comes from a proper site survey, not from a floor plan. Speaking with a designer who can work through the numbers with you before any structural decisions are made is always the right first step.

Know how wide is a spiral staircase… Now design yours around your space!

A spiral staircase is much more than a code-compliant element: stairs can be the detail that makes your spaces truly unique. Once the dimensions are clear, the real work begins, finding the configuration, the material, and the form that make sense for your interior. 

Choose Officine Sandrini for stairs with an exclusive character: fill out the form to receive our brochure free of charge and discover how to create a space tailored, elegant and without compromises!