Spiral & helical stair tool
Spiral Staircase Calculator
Riser count, degrees per tread, walk-line tread width, diameter and headroom for a spiral staircase — worked from your floor-to-floor height. Free, no account needed.
Finished floor to finished floor.
7-7.5in is comfortable; risers are equalised.
Horizontal depth, nosing excluded.
Plumb from the nosing line to the ceiling above.
Landings, stringers & handrail
Half-landings or quarter-turns in this flight.
16in suits most treads; 12in for thin or composite.
Measured above the nosing line.
4in (100mm) sphere rule.
Sets the bottom stringer drop.
Your flight
15 risers × 7.33in
14 treads at 10.50in / 26.7cm · 36.8 degree pitch
- Riser height (equal)
- 7.33in / 18.6cm
- Tread run (going)
- 10.50in / 26.7cm
- Total run
- 147.00in / 373.4cm
- Cut line (diagonal)
- 183.60in / 466.3cm
- Stringer stock length
- 195.60in / 496.8cm (16.3ft)
- Pitch
- 36.8 degrees
- 2R + G
- 25.17in / 63.9cm
- Stringers
- 4 at 12.0in centres (65.2ft stock)
- Handrail length
- 207.60in / 527.3cm (17.3ft)
- Balusters
- 39 at 5.2in centres
Code & comfort checks
IRC (US) allows a maximum 7.75in riser. Every riser in a flight must match within 3/8in.
Minimum going is 10.00in. Shallow treads are the single biggest cause of descending falls.
Comfortable domestic flights sit between 30 and 37 degrees; 42 degrees is the absolute ceiling.
Two risers plus one going should land between 24in and 25in (600-640mm) for a natural stride.
IRC (US) expects at least 36.0in of clear width above the handrail height.
Minimum headroom is 80in measured plumb from the tread nosing line.
IRC wants a graspable handrail 34-38in above the nosing line, continuous for the full flight, and no gap that passes a 4in sphere.
Guidance only — confirm against your local building control before you cut or pour.
Compare riser counts
| Risers | Riser | Going | Run | Pitch |
|---|---|---|---|---|
| 14 | 7.86in | 10.50in | 11.4ft | 38.9° |
| 15 | 7.33in | 10.50in | 12.3ft | 36.8° |
| 16 | 6.88in | 10.50in | 13.1ft | 34.9° |
The highlighted row is your current flight. Nudging the riser count is usually the cheapest way to bring a steep or shallow stair back inside code.
Advanced layout
Mark-out schedule, framing deductions & take-off
Step-by-step mark-out
| Step | Height above floor | Run from first riser |
|---|---|---|
| 1 | 7.33in / 18.6cm | 10.50in / 26.7cm |
| 2 | 14.67in / 37.3cm | 21.00in / 53.3cm |
| 3 | 22.00in / 55.9cm | 31.50in / 80.0cm |
| 4 | 29.33in / 74.5cm | 42.00in / 106.7cm |
| 5 | 36.67in / 93.1cm | 52.50in / 133.3cm |
| 6 | 44.00in / 111.8cm | 63.00in / 160.0cm |
| 7 | 51.33in / 130.4cm | 73.50in / 186.7cm |
| 8 | 58.67in / 149.0cm | 84.00in / 213.4cm |
| 9 | 66.00in / 167.6cm | 94.50in / 240.0cm |
| 10 | 73.33in / 186.3cm | 105.00in / 266.7cm |
| 11 | 80.67in / 204.9cm | 115.50in / 293.4cm |
| 12 | 88.00in / 223.5cm | 126.00in / 320.0cm |
| 13 | 95.33in / 242.1cm | 136.50in / 346.7cm |
| 14 | 102.67in / 260.8cm | 147.00in / 373.4cm |
Deduct one tread thickness so the first riser matches the rest.
Taken off the top of the stringer when the top tread is the finished floor.
Horizontal distance before the ceiling opening clears the nosing line.
Trimmer-to-trimmer opening in the floor above to keep headroom legal.
Includes 12in of extra length per board for the top and bottom cuts.
Board, stone or laminate for the tread faces including nosings.
Paint, stain, carpet or stone coverage for the whole flight.
Includes a 10% waste margin; refine it in the runner calculator.
Like these numbers? Save this flight, attach it to a client project and quote it with a free Riser account.
Continue in Riser — freeQuick answer
How do you calculate spiral staircase dimensions?
To size a spiral staircase, divide the floor-to-floor height by a riser of 180-220mm to get a whole number of risers, then divide 360 degrees (or the walk angle you need) by that number of treads to get the degrees per tread. A 1500mm diameter spiral with a 150mm centre column gives a 675mm tread radius, so a 30-degree tread is about 350mm wide at the walk line, 270mm from the centre of the flight. Standard domestic diameters are 1200mm, 1400mm, 1600mm and 2000mm.
- Risers = floor-to-floor height / target riser (180-220mm), rounded to a whole number.
- Degrees per tread = total rotation / number of treads (treads = risers - 1 on most spirals).
- Walk line sits 300mm in from the outer handrail; measure the going there, not at the column.
- Headroom of 2000mm (UK) or 1980mm (US) is the figure that usually forces a bigger diameter.
- Clear width = (diameter / 2) - centre column radius - handrail intrusion.
- Common kit diameters: 1200, 1400, 1600, 1800, 2000mm.
Work the height first, then the circle
A spiral staircase is designed in two passes. The first is vertical and identical to any straight flight: total floor-to-floor height divided by a comfortable riser gives a whole number of equal risers. The second is rotational and is what makes a spiral different — you decide how far the flight has to turn to arrive facing the right way, then split that rotation evenly between the treads. Get the vertical pass right and the stair is safe; get the rotational pass right and it is pleasant to walk.
| Floor-to-floor | Risers | Riser height | Treads | Rotation at 30 deg/tread |
|---|---|---|---|---|
| 2400mm | 12 | 200mm | 11 | 330 degrees |
| 2600mm | 13 | 200mm | 12 | 360 degrees |
| 2800mm | 14 | 200mm | 13 | 390 degrees |
| 3000mm | 15 | 200mm | 14 | 420 degrees |
| 3200mm | 16 | 200mm | 15 | 450 degrees |
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorDiameter, centre column and the usable tread
The tread of a spiral is a wedge. Its width at any point is the radius at that point multiplied by the tread angle in radians. On a 1600mm diameter spiral with a 150mm column, the outer radius is 800mm and the walk line sits at roughly 500mm from the centre; a 30-degree tread is then about 262mm at the walk line and 419mm at the outer edge. That walk-line figure is the number to compare against a straight stair's going, and it is why a 1200mm spiral feels tight even when the drawing says the treads are wide.
| Diameter | Outer radius | Walk-line radius | 30 deg tread at walk line | Best for |
|---|---|---|---|---|
| 1200mm | 600mm | 300mm | 157mm | Loft or occasional access |
| 1400mm | 700mm | 400mm | 209mm | Secondary flight |
| 1600mm | 800mm | 500mm | 262mm | Comfortable main flight |
| 1800mm | 900mm | 600mm | 314mm | Main flight, easy carrying |
| 2000mm | 1000mm | 700mm | 366mm | Feature stair, furniture access |
Headroom is the constraint that catches people out
On a spiral, the ceiling is rarely the problem — the tread above your head is. The vertical clearance between one tread and the tread directly overhead equals the riser height multiplied by the number of treads in a full turn. With 200mm risers and 12 treads per 360 degrees, that is 2400mm of clearance, comfortably above the 2000mm minimum. Increase the rotation per tread and the number of treads per turn drops, taking the clearance with it, so a very open spiral can fail headroom even with generous risers.
- Measure finished floor to finished floor, including the thickness of both floor coverings.
- Divide by 200mm and round to the nearest whole number to fix the riser count.
- Subtract one for the tread count, because the top landing acts as the final step.
- Decide the arrival direction and set the total rotation, usually 270 to 360 degrees.
- Divide rotation by tread count and check the result sits between 22 and 36 degrees.
- Check the walk-line going against the tread width table above.
- Check tread-to-tread vertical clearance is at least 2000mm.
- Size the floor opening at diameter plus 50 to 100mm all round.
Fitting a runner or carpet to a spiral
A spiral cannot take a continuous runner: every tread is a wedge, so each step needs its own cut piece bound on the exposed edge, or a bespoke tapered runner made to the exact angle. Work out the material by treating each tread as a trapezoid — average the walk-line width and outer width, multiply by the tread depth plus riser height, and add generously for binding and waste. If you want the numbers for a straight flight instead, the stair runner calculator on the home page handles that in one pass.
Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.
Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.
Frequently asked questions
What diameter does a spiral staircase need to be?
1200mm is the smallest usable domestic spiral and suits a loft or secondary access only. 1400mm to 1600mm is the comfortable range for a main flight, and 1800mm to 2000mm is what you want if furniture has to travel up it. Diameter drives the tread width at the walk line far more than the tread count does, so if the steps feel narrow the fix is a wider circle, not more treads.
How many steps does a spiral staircase have?
Divide the finished floor-to-floor height by a riser between 180mm and 220mm and round to a whole number. A typical 2600mm floor-to-floor gives 13 risers at 200mm, which is 12 treads plus the top landing platform. Every riser on a spiral must be identical — a spiral is unforgiving of the odd short step at the bottom.
How much rotation should a spiral staircase have?
Most domestic spirals turn between 270 and 360 degrees over the flight. Divide the rotation by the number of treads: 12 treads over 360 degrees is 30 degrees per tread, which is the usual comfortable figure. Below roughly 22 degrees per tread the steps feel cramped at the walk line; above 36 degrees they feel like a ladder wrapping around the pole.
What is the walk line on a spiral stair?
An imaginary circle 300mm inside the outer handrail where the going is measured, because that is where feet actually land. A tread measured at the centre column looks impossibly narrow and measured at the outer edge looks generous; only the walk line reflects how the stair walks. Building control checks the going there too.
What headroom does a spiral staircase need?
2000mm in the UK and 1980mm under the IRC, measured vertically from the tread nosing to the underside of the floor opening or the tread directly above. On a tight spiral the constraint is usually the tread above rather than the ceiling, so it is worth checking the vertical gap between successive treads: rotation and riser height together decide it.
How big does the floor opening need to be?
Allow the stair diameter plus roughly 50-100mm of clearance all round for the handrail sweep and finishing trim. A 1600mm spiral therefore wants a 1700mm square or round opening. Confirm the kit manufacturer's opening figure before the joists are trimmed, because it varies with the balustrade type.
Is this spiral staircase calculator free?
Yes. Rise, run, riser count, pitch and code checks are free with no sign-up. A free Riser account only matters if you want the flight saved into a project so the numbers can be reused in a quote or shared with a client.
Related tools & guides
Start with the numbers: our stair runner calculator also works as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length first time.