Spiral Staircase Calculator (steps, rise, rotation & IRC code check)

Understanding the spiral staircase calculator — concept and principle

This calculator lays out a spiral staircase from the floor-to-floor height, the stair diameter, the centre column and the rotation of each tread. It gives the number of risers, the rise per step, the tread depth where people actually walk, the clear width and the headroom, and checks them against the spiral-stair limits of the US International Residential Code (IRC).

Tread depth is measured on the dashed walkline; all IRC limits are met here.

How it is calculated

Risers = height ÷ 241.3 mm (9½ in), rounded up

Rise per step = height ÷ risers; treads = risers − 1

Walkline radius = column diameter ÷ 2 + 304.8 mm (12 in)

Tread depth at the walkline = walkline radius × angle per tread in radians

Clear width = (stair diameter − column diameter) ÷ 2

Headroom = rise per step × (360° ÷ angle) − tread thickness

The upper floor serves as the last step, so there is one tread fewer than risers.

The principle

A spiral stair is a straight stair wound around a post. Each tread is a wedge, narrow at the column and wide at the rim, so its depth depends on where you measure. The code measures it on a walkline 12 in from the column edge, roughly where a foot lands. The arc length of a wedge at radius r is r × angle (in radians). Headroom is limited by the tread directly above your head: after one full turn you have climbed 360° ÷ angle steps, minus the thickness of the tread above. IRC R311.7.10.1 asks for a clear width of at least 26 in (660 mm), tread depth at least 6¾ in (171 mm) at the walkline with a walkline radius of at most 24½ in (622 mm), rise at most 9½ in (241 mm), identical treads, and headroom of at least 6 ft 6 in (1,981 mm).

Worked example

Default inputs: height 2,700 mm, diameter 1,600 mm, column 110 mm, 30° per tread, tread thickness 40 mm.

Risers = 2,700 ÷ 241.3 = 11.19, rounded up to 12; rise per step = 2,700 ÷ 12 = 225 mm.
Treads = 11; total turn = 11 × 30° = 330°.
Walkline radius = 55 + 304.8 = 359.8 mm; depth = 359.8 × 0.5236 = 188 mm.
Clear width = (1,600 − 110) ÷ 2 = 745 mm.
Headroom = 225 × 12 − 40 = 2,660 mm. All limits are met: yes.

Each point is the calculator re-run with only “Floor-to-floor height (mm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Floor-to-floor height (mm)	Risers
675	3
1,350	6
2,025	9
2,700	12
3,375	14
4,050	17
5,400	23

How to read the result

Each "no" points to the input to change. Too little tread depth or headroom usually means the angle per tread is too large; a smaller angle gives more treads per turn, deeper treads and more headroom, but uses more turn to reach the landing. Too little clear width needs a larger diameter.

Common mistakes

Measuring height from floor finish to subfloor instead of finished floor to finished floor.
Forgetting that the handrail reduces clear width.
Counting the upper floor as a tread.

Keep in mind: This is a planning aid based on one model code. Real stairs must follow your local building code, which may differ, and should be designed or checked by a qualified professional.

Frequently asked questions

What is the minimum diameter for a code spiral stair?

With a 26 in clear width and a slim column, the diameter is a little over 52 in, about 1.35–1.4 m. Handrails push it higher.

Why is the rise allowed to be higher than on normal stairs?

The IRC allows 9½ in for spiral stairs because they are compact secondary stairs; ordinary residential stairs are limited to 7¾ in.

Sources: International Code Council, International Residential Code (2018), Section R311.7.10.1 "Spiral stairways"

Checked 9 Nov 2026. International Code Council, International Residential Code (2018 edition), Section R311.7.10.1 “Spiral stairways” (clear width ≥ 26 in, tread depth ≥ 6¾ in at the walkline, walkline radius ≤ 24½ in, rise ≤ 9½ in, headroom ≥ 6 ft 6 in, identical treads).

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