StairGen Pro › Winder Stair Calculator (90° L-Shaped Winders)
Guide & codes
This winder stair calculator designs an L-shaped stair that turns 90° on winder treads instead of a landing. The winders fill the corner, so the stair fits in less space than an L-shaped stair with a landing, which is why quarter-turn winder stairs are so common in terraced houses, cottages and loft conversions.
Enter the total rise (Y), the run of the upper flight (X), the width (W), the steps in each flight (P and Q) and how many winders make up the turn (K). The calculator divides the 90° between the winders, checks comfort and outputs each winder tread as a separate part, with its own shape, ready to cut. With K = 3 you get the classic three-winder turn whose middle tread UK joiners call a kite winder.
When to choose this stair
Choose the 90° winder stair when there is no room for a landing: the 3 winders fill the corner and turn while climbing, so the L-shaped stair fits in less floor area. It is the most common wooden L-shaped stair in small houses, especially up to a loft or an attic.
The catch is on the newel side, where the going of each winder narrows sharply and where most trips happen. Put the handrail on the outer side, where people walk, and check the shape of each winder in the DXF, because the calculator rates comfort using the going of the straight flights.
Calculator parameters
Each dimension carries the letter used in the editor and on the drawings.
- YTotal rise
- default 2,600 mm · 500 to 8,000 mm
- Height from the lower floor to the finished upper floor.
- XTotal run
- default 3,250 mm · 500 to 12,000 mm
- Plan length available for the flight (on turning stairs, the upper flight).
- WStair width
- default 1,000 mm · 500 to 2,500 mm
- Clear width of the flight.
- ZTread thickness
- default 50 mm · 3 to 120 mm
- Thickness of the tread (board or plate).
- FNosing
- default 50 mm · 0 to 100 mm
- How far each tread overhangs the one below.
- PSteps, lower flight
- default 4 · 1 to 25
- Steps before the turn.
- KWinder steps
- default 3 · 2 to 6
- Winder treads in each turn.
- QSteps, upper flight
- default 8 · 2 to 25
- Steps after the turn, up to the top.
- TStringer thickness
- default 50 mm · 20 to 120 mm
- Thickness of the stringer board.
How it's calculated
The total rise Y = 2,600 mm is shared between 15 risers (the 4 steps below, the 3 winders and the 8 steps above): 173.3 mm each. On the straight flights the run is 281 mm and the tread depth 331 mm.
With K = 3 winders, the 90° turn is divided into equal angles and every winder has a different shape, which is why the bill of materials lists them separately (E2, E3…) and the DXF gives each one its own sheet. With the default values the step formula is 628 mm and the pitch 31.6°: “Comfortable stair”.
If the winders become too narrow on the inside, increase K or the width W: the treads in the turn open up and the going on the walkline grows. A newel post at the inner corner carries the winder stringers, and it appears in the parts list with its own mark.
Result with the default values
Comfort verdict: Comfortable stair.
Complies with IRC / IBC
| Step formula 2R + G | 628 mm | 600 – 660 mm |
| Tread depth | 331 mm | ≥ 280 mm |
| Pitch | 31.6° | 30° – 40° |
| Riser | 173 mm | ≤ 197 mm |
| Going | 281 mm | ≥ 254 mm |
| Clear width | 1,000 mm | ≥ 914 mm |
| Guarding height | 900 mm | ≥ 864 mm |
Steps
| Risers | 15 pcs |
| Riser height | 173 mm |
| Run (going) | 281 mm |
| Tread depth (run + nosing) | 331 mm |
| Open riser gap | 123 mm |
| Pitch | 31.6° |
| Nosing-to-nosing distance | 330 mm |
Wood
| Tread area | 4.587 m² |
| Winder tread angle | 30.0° |
Building codes
Winders are allowed in dwellings in the US, the UK, Australia and Canada, with their own rules. Under the IRC, a winder must have at least 10 in (254 mm) of tread depth at the walkline, 12 in (305 mm) from the narrow side, and at least 6 in (152 mm) at any point. Approved Document K measures the going on the centre line when the stair is narrower than 1 m (at 270 mm from each side when it is wider) and asks for at least 50 mm at the narrow end. Commercial stairs under the IBC may not use winders. The Australian rule that limits winders to three in place of a quarter landing should be checked in the current NCC tables before relying on it.
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- Z1StringerBoard 50 × 2
- Z2Winder stringerBoard 50 × 2
- Z3StringerBoard 50 × 2
- E1Treadt=50 × 12
- E2Treadt=50 × 1
- E3Treadt=50 × 1
- E4Treadt=50 × 1
- P1PostTimber 100×100 × 1
- T1Risert=20 × 12
- B1HandrailRound timber Ø50 × 1
- B2BalusterBatten 35×35 × 12
- B3HandrailRound timber Ø50 × 1
Frequently asked questions
Are winder stairs legal?
Yes, in homes, as long as the winders meet the tapered-tread rules of your code: in the US a 10 in tread at the walkline and 6 in at the narrow end; in England a going measured on the centre line and at least 50 mm at the narrow end. In commercial buildings they are generally not allowed.
How many winders in a 90° turn?
Usually three, the kite-winder layout. Fewer winders make the treads very narrow on the inside; more winders take up more space. Change K and watch how the tread depth and the verdict change.
Winder stairs or a landing?
Winders save space and about one step's worth of floor area; a landing is more comfortable and safer for children and older people. With the same opening, try both calculators and compare.
What is a kite winder?
The middle tread of a three-winder quarter turn, so called because its four-sided shape looks like a kite. The treads on either side of it are triangular or tapered.