StairGen Pro › Stair Stringer Calculator (Cut Stringers)
Guide & codes
This stair stringer calculator works out cut stringers: boards notched into a sawtooth so that each tread sits on top of a tooth (open stringer in North America, cut string in the UK). Enter your total rise and total run and you get the number of risers and treads, the riser height, the run, the stringer length and a dimensioned layout diagram of the stringer, plus a 3D model of the whole stair.
Unlike a closed stringer stair, the treads here show their ends and can overhang the stringers. The step layout is the same as on the straight wooden stair, and the DXF gives you each cut stringer at full size, in millimetres or inches, so you can print it as a template or send it to a CNC router instead of marking it out by hand.
When to choose this stair
Choose the cut-stringer stair when you want an open wooden stair with the stepped profile on show: the stringer is notched into teeth and each tread rests on top. It is the typical porch, deck or basement stair, and the simplest joinery stair to lay out.
Compared with the stair with board stringers, the notches remove timber: from the same board a cut stringer is weaker, so it usually needs a deeper section. The step layout does not change: the same height, length and number of steps give the same riser, the same going and the same pitch (31.8°).
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 4,200 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.
- CNumber of steps
- default 15 · 2 to 40
- Steps in the flight, including the top one.
- TCut stringer thickness
- default 50 mm · 20 to 120 mm
- Thickness of the stringer board.
How it's calculated
The total rise Y = 2,600 mm over 15 risers gives 173.3 mm per step; the run X = 4,200 mm over 15 steps gives a run of 280 mm and a tread depth of 330 mm with the nosing. Step formula: 627 mm. Pitch: 31.8°. Verdict: “Comfortable stair”.
The notches reproduce the rise and run of every step, so the stringer sheet in the DXF doubles as a cutting template. The board is 50 mm thick by default. The Data tab gives the nosing-to-nosing distance along the pitch line: multiply it by the number of treads and add the top and bottom cuts to get the stringer length you need to buy.
The calculator draws the two outer stringers. On wider stairs, carpenters usually add a third stringer in the middle; it is cut exactly like the outer ones.
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 | 4.950 m² |
Building codes
The rules are those of any straight stair: 7¾ in (196 mm) maximum rise and 10 in (254 mm) minimum tread under the IRC, 220 mm rise and 42° pitch under Approved Document K. Cutting the notches removes wood, so the depth left below the notches must be checked against the span and the load. For outdoor deck stairs, check the stringer rules in your local code or the deck construction guide your inspector uses.
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- Z1Cut stringerBoard 50 × 2
- E1Treadt=50 × 15
- T1Risert=20 × 15
- B1HandrailRound timber Ø50 × 1
- B2BalusterBatten 35×35 × 15
Frequently asked questions
How do you calculate stair stringers?
Divide the total rise by the number of risers to get the riser height, and the total run by the number of treads to get the run per step (there is always one tread fewer than risers when the top step is the upper floor). The stringer length is the hypotenuse: the square root of total rise squared plus total run squared, plus a little for the top and bottom cuts.
How long is a 2x12 for a 7-step stringer?
With 7 risers of 7¼ in (50¾ in of total rise) and 6 treads of 10 in (60 in of run), the pitch line is √(50.75² + 60²) ≈ 78.6 in, about 6 ft 7 in, so an 8 ft 2x12 is enough. If you mean seven treads and eight risers, the line is about 91 in (7 ft 7 in): an 8 ft board leaves almost nothing for the cuts, so buy a 10 ft one.
Should stair stringers be 2x10 or 2x12?
For cut stringers, 2x12 is the usual choice: once the notches are cut, a 2x10 keeps little wood below them. A 2x10 can work for a closed stringer, which is not notched. In metric terms, the default here is a 50 mm board; the right depth depends on span and load.
How do you mark out a cut stringer?
Traditionally with a framing square, setting the rise on one arm and the run on the other and stepping it along the board tooth by tooth. With the DXF you can skip that: the stringer sheet carries the complete profile at full size.