StairGen Pro › Three-Flight Stair Calculator with Two Landings
Guide & codes
This three-flight stair calculator designs a wooden stair that turns twice by 90° on two square quarter landings and wraps around three sides of the stairwell. It is also known as a three-quarter turn stair, or a U-shaped stair with two landings, and it is the natural fit for an almost square opening.
You set the total rise (Y), the run of the upper flight (X), the width (W) and the steps in the lower (P), middle (M) and upper (Q) flights. The calculator places both landings, checks comfort and lists stringers, landing frames, posts, treads, risers and railing.
When to choose this stair
Choose the three-flight stair with landings when the opening is almost square and the stair has to wrap around it: three flights of 4, 4 and 5 steps and two 90° turns on landings. It is the classic stair around a stairwell or a light well, and the most comfortable option when neither an L nor a U fits.
Compared with the three-flight winder stair, the two landings take more floor area but give two resting points and no narrow treads. Sharing the steps among the three flights moves the landings up or down without changing the riser; the going is set by the length of the upper flight (X = 2,400 mm).
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 4 · 1 to 25
- Steps before the turn.
- MSteps, middle flight
- default 4 · 1 to 25
- Steps between the two turns.
- QSteps, upper flight
- default 5 · 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: 4 steps below, a landing, 4 in the middle flight, another landing and 5 above. Each step rises 173.3 mm.
On the upper flight, X = 2,400 mm minus the landing width leaves a run of 280 mm and a tread depth of 330 mm. Step formula: 627 mm; pitch: 31.8°; verdict: “Comfortable stair”.
Each landing is a square as wide as the stair, with its own frame of timber beams. Balancing the steps between the three flights (P, M, Q) is what makes the stair fit the opening without touching the tread depth: a longer middle flight widens the stair, while longer outer flights make it deeper.
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 | 3.861 m² |
Building codes
With two landings no flight climbs very far, which makes per-flight limits easy to meet: the IRC allows up to 151 in (3835 mm) of rise per flight and the NCC up to 18 risers. Each landing must be at least as wide as the stair, and headroom must be checked where the upper flight passes under the floor above.
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
- Z2StringerBoard 50 × 2
- Z3StringerBoard 50 × 2
- E1Treadt=50 × 13
- M1Landing frameBeam 50×150 × 4
- M2Landing frameBeam 50×150 × 4
- R1Landingt=50 × 2
- P1PostTimber 100×100 × 1
- P2PostTimber 100×100 × 1
- T1Risert=20 × 13
- B1HandrailRound timber Ø50 × 2
- B2BalusterBatten 35×35 × 13
- B3HandrailRound timber Ø50 × 1
Frequently asked questions
What is a three-flight stair?
A stair that climbs in three flights with two 90° turns, usually with a quarter landing at each turn. It is often listed as a three-quarter turn stair, a name some builders also use for stairs that turn 270°; what defines this one is three flights and two quarter landings.
When does a three-flight stair make sense?
When the opening is nearly square and fits neither an L-shaped nor a U-shaped stair: the three flights wrap around the well and the stair stays compact in both directions.
Can it have winders instead of landings?
Yes: the double winder staircase calculator replaces both landings with winders and saves some floor area.