Stair calculator
Risers, treads, total run and stringer length for a straight stair
Type the height from finished floor to finished floor and choose the code your defaults should follow. The example is a 9 ft (108 in) rise under the IRC.
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- Each riser
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- Treads
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- Total run
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- Angle
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- Stringer length (along the pitch line)
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- Headroom the code asks for
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A layout from the sizes you typed, not a design approval. Stairs are covered by building codes, and local rules decide: check your plan with your local building authority or a qualified person before you cut anything.
How the stair is laid out
- Number of risers = total rise / largest riser allowed, rounded up to a whole number.
- Each riser = total rise / number of risers, so every step is the same height.
- Treads = risers − 1, because the top step is the upper floor itself.
- Total run = treads x tread depth.
- Stringer length = √(total rise² + total run²), along the pitch line. The board you cut it from must be longer.
Worked example (IRC)
A 108 in rise: 108 / 7.75 = 13.9, so 14 risers of 7.714 in (about 7 11/16 in). That gives 13 treads; at 10 in each the total run is 130 in (10 ft 10 in). The stringer's pitch line is √(108² + 130²) = 169.0 in, and the stair rises at 37.6°.
The code limits
The defaults follow the US International Residential Code (IRC): risers no taller than 7 3/4 in and treads at least 10 in deep. The other choices load the limits for private stairs in Canada's National Building Code, England's Approved Document K and Australia's National Construction Code. Your state, province or council may adopt a different edition or add its own rules, and codes also set width, headroom, nosings, handrails, guards and landings. Local rules decide.
This page does not design the structure or judge whether a stair is safe. Ask your local building authority or a qualified person.
Defaults and where they come from
Every default is only a starting point. Change any box to match what you are buying or building.
| Default | Value | Source |
|---|---|---|
| IRC (US) | Riser at most 7 3/4 in; tread at least 10 in; headroom at least 6 ft 8 in | International Residential Code 2021, R311.7.2 and R311.7.5. States and cities adopt different editions. |
| NBC (Canada) | Rise 125 to 200 mm; run 255 to 355 mm; private stairs | National Building Code of Canada 2020, Tables 9.8.4.1 and 9.8.4.2. Provinces adopt their own codes. |
| Approved Document K (England) | Rise 150 to 220 mm; going 220 to 300 mm; pitch at most 42°; 2 x rise + going 550 to 700 mm; private stair | Approved Document K (2013), section 1 and Table 1.1. Wales, Scotland and Northern Ireland have their own rules. |
| NCC (Australia) | Riser 115 to 190 mm; going 240 to 355 mm; 2 x riser + going 550 to 700 mm | National Construction Code 2022, Housing Provisions Table 11.2.2a (stairs other than spiral). |
| Waste allowance | None | This page lays out a stair; it does not order material. |