The three numbers that define a spiral
Every spiral kit sold is described by its diameter, and that single figure decides more about how the stair feels than anything else. Diameter sets the radius, the radius sets how wide a tread is at the point where your foot lands, and that width is what your body reads as comfort or as danger. A 1200mm spiral and a 1800mm spiral can have identical riser heights and identical tread angles and still walk completely differently, because at the walk line one gives you 157mm of tread and the other gives you 314mm.
The second number is riser height. Spirals follow the same rules as straight flights: 180mm to 220mm domestic, every riser identical, no exceptions for the bottom step. Divide the finished floor-to-floor height by 200mm, round to a whole number, and you have your riser count. Subtract one and you have the tread count, because the upper floor acts as the last step.
The third is rotation. Most domestic spirals turn between 270 and 360 degrees over the flight, chosen so the user arrives facing the right way at the top. Divide that rotation by the tread count and you get degrees per tread, which should land between 22 and 36 for a flight people use daily.
| Diameter | Walk-line radius | 30 deg tread at walk line | Typical use |
|---|---|---|---|
| 1200mm | 300mm | 157mm | Loft or occasional access |
| 1400mm | 400mm | 209mm | Secondary flight |
| 1600mm | 500mm | 262mm | Comfortable main flight |
| 1800mm | 600mm | 314mm | Main flight, easy carrying |
| 2000mm | 700mm | 366mm | Feature stair |
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 calculatorHeadroom on a spiral is measured tread to tread
On a straight flight, headroom is the distance from a nosing to the ceiling or the trimmer above. On a spiral, the thing above your head is usually another tread of the same stair. The clearance equals the riser height multiplied by the number of treads in a full 360-degree turn. With 200mm risers and 12 treads per turn you get 2400mm, comfortably over the 2000mm UK minimum. Open the stair out to 40 degrees per tread and you only have nine treads per turn, which drops the clearance to 1800mm and fails.
Quick check
Tread-to-tread clearance = riser height x (360 / degrees per tread). If that number is under 2000mm, tighten the tread angle or reduce the riser.
Sizing the floor opening
Allow the stair diameter plus 50mm to 100mm all round for the handrail sweep and the trim. A 1600mm spiral therefore wants a 1700mm to 1800mm opening. The opening can be square or round; square is easier to trim into joists and gives a little extra elbow room at the top where people step off. Always confirm the figure with the kit manufacturer before the joists are cut, because balustrade type changes the sweep.
- Measure finished floor to finished floor, including both floor coverings.
- Fix the riser count before you choose the diameter — height is not negotiable, diameter often is.
- Check the arrival direction at the top; rotation follows from it.
- Leave the walk-line tread width above 250mm for a flight in daily use.
- Confirm the floor opening with the supplier, not with the drawing.
Covering a spiral
A spiral cannot take a continuous runner: each tread is a wedge, so each one needs an individually cut and bound piece, or a bespoke tapered runner made to your exact tread angle. Estimate material by treating each tread as a trapezoid, averaging the walk-line and outer widths, then adding the riser height and a generous binding allowance. Straight flights are far simpler, and the stair runner calculator on the home page handles those 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 is the minimum diameter for a spiral staircase?
1200mm is the practical minimum for domestic use and is really only suitable for a loft or secondary access. Below that the tread width at the walk line drops under 160mm, which is narrower than a foot, and the stair becomes genuinely unsafe to descend carrying anything.
How many steps are in a spiral staircase?
Divide the floor-to-floor height by a riser between 180mm and 220mm. A 2600mm floor-to-floor gives 13 risers at 200mm, so 12 treads plus the landing. Every riser must be identical, including the first and last.
How many degrees should each spiral tread turn?
Between 22 and 36 degrees for a flight in daily use, with 30 degrees the common default. Fewer degrees per tread means more treads per turn, more headroom clearance and a gentler walk, at the cost of more rotation over the flight.
Do spiral staircases meet UK building regulations?
Yes, when they satisfy Approved Document K: risers of 150mm to 220mm, a going measured at the walk line of at least 145mm for private stairs, 2000mm headroom, and a guard no gap of which passes a 100mm sphere. A spiral serving as the only access to a habitable room is judged more strictly than a secondary flight.
How big should the floor opening be?
Stair diameter plus 50mm to 100mm all round. A 1600mm spiral wants roughly a 1700mm to 1800mm opening. Confirm the exact figure with the kit supplier before the joists are trimmed.
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.
Size a flight with the spiral staircase calculatorEvery guide is written from hands-on measuring and fitting experience, checked against the calculator’s own maths, and re-reviewed whenever prices or methods change. Read our editorial standards.