StairGen Pro › Double Winder Staircase Calculator
Guide & codes
This double winder staircase calculator designs a three-flight wooden stair that makes two 90° turns on winder treads, with no landings. In the UK this is what a double winder stair means: two quarter turns with a short straight flight between them. It is the most compact of the stairs that wrap around a square opening, and a common choice for loft conversions and narrow houses.
Enter the total rise (Y), the run of the upper flight (X), the width (W), the steps in each flight (P, M and Q) and the winders in each turn (K). The calculator lays out both sets of winders and outputs every winder tread as its own part. If the opening allows it, the three-flight stair with two landings is more comfortable; if you are thinking of a single 180° turn around one newel, see the U-shaped winder stair instead.
When to choose this stair
Choose the three-flight winder stair when the opening is small and almost square: each 90° turn is made with 3 winders and no landing, so the stair climbs further in less floor area than any other with straight flights. It is the usual way up to an attic or a mezzanine through an opening of about 2 × 2 m.
The price is comfort: with the default values the verdict is “Comfortable stair”, and the winders narrow next to the posts. If there is room, the version with landings is much safer; if not, check the going of each winder in the DXF, which the calculator does not check, and fit a continuous handrail on the outer side.
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 2,400 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 2 · 1 to 25
- Steps before the turn.
- MSteps, middle flight
- default 2 · 1 to 25
- Steps between the two turns.
- QSteps, upper flight
- default 5 · 2 to 25
- Steps after the turn, up to the top.
- KWinder steps
- default 3 · 2 to 6
- Winder treads in each turn.
- 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: 173.3 mm per step. On the straight flights the run is 280 mm and the tread depth 330 mm with the nosing F = 50 mm.
Each turn has K = 3 winders sharing the 90°. Step formula: 627 mm; pitch: 31.8°; verdict: “Comfortable stair”. In such a small opening it is easy to end up with a short tread: the Summary panel tells you how far to extend the nosing or how many steps to remove to fix it.
The middle flight (M) sets the width of the stair as a whole, and the two outer flights (P and Q) its depth. Moving one step from P to M, or from M to Q, is often enough to make the stair fit without changing the riser height.
Result with the default values
Comfort verdict: Comfortable stair.
Complies with IRC / IBC
| Step formula 2R + G | 627 mm | 600 – 660 mm |
| Tread depth | 330 mm | ≥ 280 mm |
| Pitch | 31.8° | 30° – 40° |
| Riser | 173 mm | ≤ 197 mm |
| Going | 280 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) | 280 mm |
| Tread depth (run + nosing) | 330 mm |
| Open riser gap | 123 mm |
| Pitch | 31.8° |
| Nosing-to-nosing distance | 329 mm |
Wood
| Tread area | 4.692 m² |
| Winder tread angle | 30.0° |
Building codes
Each set of winders must meet the tapered-tread rules: 10 in (254 mm) at the walkline and 6 in (152 mm) at any point under the IRC; a going measured on the centre line and at least 50 mm at the narrow end under Approved Document K. With two turns in a small space, also check the headroom under the floor above: 2.0 m in England or, in a loft conversion with limited space, 1.9 m at the centre of the stair and 1.8 m at the side.
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 × 4
- Z3StringerBoard 50 × 2
- Z4StringerBoard 50 × 2
- E1Treadt=50 × 9
- E2Treadt=50 × 2
- E3Treadt=50 × 2
- E4Treadt=50 × 2
- P1PostTimber 100×100 × 1
- P2PostTimber 100×100 × 1
- T1Risert=20 × 9
- B1HandrailRound timber Ø50 × 2
- B2BalusterBatten 35×35 × 9
- B3HandrailRound timber Ø50 × 1
Frequently asked questions
What is a double winder staircase?
A stair with two 90° winder turns and a short flight between them, so it changes direction twice without a landing. It is different from a 180° winder, where a single group of winders turns half a circle around one newel.
What are typical double winder staircase dimensions?
It is designed for small, roughly square openings: the default here is an opening 2,400 mm long for a 1,000 mm wide stair and a rise of 2,600 mm. The Summary panel gives the exact footprint, and it changes as you move steps between P, M and Q.
How many winders in each turn?
Three per 90° turn is usual, which is the default (K = 3). More winders open up the turn but take up more floor area.
How do I see the shape of each winder?
In the Drawings tab, by clicking its mark (E2, E3…): every winder has its own sheet with dimensions, and in the DXF it sits on the TREADS layer, ready to cut.