StairGen Pro › Spiral Staircase Calculator
Guide & codes
This spiral staircase calculator lays out treads fanning around a steel centre column, with a helical handrail and balusters. Enter the floor-to-floor height (H), the outside diameter (D1), the column diameter (D2) and the total rotation (A), and it works out the number of steps, the riser height, the tread angle and the going on the walkline, then draws every tread so it can be cut.
A spiral is the stair that takes the least floor space (a 2,000 mm circle with the default values) and also the easiest one to get wrong: too few treads and every step is a climb, too small a diameter and the usable tread disappears. Here you see it in figures and in 3D before anything is cut. The calculator works in millimetres or in inches (the mm / in switch in the editor), so it doubles as a metric spiral staircase calculator; the inch equivalents of the code limits are also listed below.
When to choose this stair
Choose a spiral staircase when nothing else fits: with the default values it sits inside a 2,000 mm circle and climbs around a centre column. It is the classic secondary access (lofts, attics, roof terraces) and the most striking piece of metalwork, but not the most comfortable stair for carrying things up.
Unlike a stair with winders, it has no straight flights: all treads are identical wedges and the going grows from the inside out. For a comfortable stair, settle the diameter and the number of treads before the rotation: a small diameter leaves a narrow way through and too few treads give a high riser.
Calculator parameters
Each dimension carries the letter used in the editor and on the drawings.
- HStair height
- default 2,800 mm · 1,000 to 8,000 mm
- Height between finished floors.
- D1Stair diameter
- default 2,000 mm · 1,000 to 3,500 mm
- Overall outside diameter.
- D2Column diameter
- default 100 mm · 50 to 400 mm
- Diameter of the centre tube.
- ATotal rotation
- default 480° · 180 to 1,080°
- Angle the stair turns from bottom to top.
- ZTread thickness
- default 50 mm · 3 to 120 mm
- Thickness of the tread (board or plate).
- CNumber of steps
- default 16 · 2 to 40
- Steps in the flight, including the top one.
How it's calculated
The height H = 2,800 mm is divided between C = 16 treads: 175.0 mm per step. The total rotation A = 480° over the same treads gives 30.0° per tread. The going is not the same across the tread: it is measured on the walkline, where people actually walk, and there it is 288 mm. The calculator places the walkline 500 mm from the edge of the column.
Each tread is as long as the outer radius minus the column radius. Its outer width (the chord) and the width of the rough blank appear in the Data tab, so you can order plate or boards and nest the treads with little waste.
On a spiral the step formula tends to come out high: with the default values, 2 × 175.0 + 288 = 638 mm (“Comfortable stair”). The fix is more treads, that is, a lower riser; the Summary panel tells you how many. More rotation with the same number of treads widens each tread and lengthens the going.
Result with the default values
Comfort verdict: Comfortable stair.
Complies with IRC / IBC
| Step formula 2R + G | 638 mm | 600 – 660 mm |
| Tread depth | 288 mm | ≥ 280 mm |
| Riser | 175 mm | ≤ 241 mm |
| Clear width | 950 mm | ≥ 660 mm |
| Guarding height | 900 mm | ≥ 864 mm |
Steps
| Riser height | 175 mm |
| Tread angle | 30.0° |
| Going on walkline (500 mm from column edge) | 288 mm |
| Open riser gap | 125 mm |
Tread
| Tread length | 950 mm |
| Tread width (outer chord) | 518 mm |
| Blank width | 536 mm |
Building codes
Spiral stairs have their own rules. Under the IRC (R311.7.10.1), a spiral stair needs at least 26 in (660 mm) of clear width, a tread depth of at least 6¾ in (171 mm) at the walkline 12 in (305 mm) from the narrow edge, a riser of no more than 9½ in (241 mm) and 6 ft 6 in (1981 mm) of headroom. Australia's NCC sets a rise of 140–220 mm, a going of 210–370 mm and 2R + G between 590 and 680 mm for spiral stairs (check the current table before relying on it). In the UK, Approved Document K refers spiral and helical stairs to BS 5395-2. Note that the IRC and this calculator measure the walkline at different points: the going at 500 mm from the column edge only feeds the comfort check, so compare the going at 305 mm yourself when building to the IRC. On a spiral, the code check in the Summary compares only the riser, the width and the guarding, using the code's own spiral rules where it has them: under the IRC, 9½ in of riser and 26 in of width (R311.7.10.1); under the NCC, a rise of 140–220 mm.
The calculator checks comfort (2R + G, going and pitch) and the building code of the country you pick in the panel (here, IRC / IBC for a dwelling): only the limits that code sets for riser, going, 2R + G, width, pitch, risers per flight and guarding. It is for guidance only: always confirm the code in force and local rules.
Building codes by country, with sourcesParts with the default values
The DXF (AutoCAD 2007, in millimetres or inches) carries these parts with their shop marks: one dimensioned sheet per part, the general arrangement and the bill of materials with weights.
What's in the DXF- E1Spiral treadt=50 × 16
- P1Centre columnTube Ø100 × 1
- B1Helical handrailTube Ø42.4 × 1
- B2BalusterRound bar Ø16 × 16
Frequently asked questions
How many steps are in a spiral staircase?
As many as it takes to keep the riser within limits: divide the floor-to-floor height by the target riser and round up. For 2.8 m (about 110 in) and the IRC maximum of 9½ in that is 12 risers; for a more comfortable rise of around 200 mm it is 14.
What size spiral staircase do I need?
Kits usually range from about 1.2 to 1.8 m (4 to 6 ft) in diameter, and made-to-measure stairs go to 2 m or more. Under the IRC, 26 in of clear width plus the column and the handrail usually means a diameter of at least 5 ft (about 1520 mm). The smaller the diameter, the shorter the going on the walkline: change D1 and watch the going and the verdict.
How do you measure for a spiral staircase?
Measure the finished floor-to-floor height and the size of the floor opening, and decide where you want to step off at the top. The opening is usually a little larger than the stair diameter, and the rotation A sets where the top tread lands: 360° is a full turn and 450° a turn and a quarter.
Spiral or helical staircase?
The terms are often used interchangeably. Strictly, a helical stair turns around an open well with no column; the one in this calculator is a spiral staircase, with every tread fixed to a centre column.