How the numbers are worked out
The total rise is divided by the maximum riser height and rounded up to a whole number of risers. Every riser is then the same height: total rise divided by the number of risers. With the upper floor as the top step, there is one tread fewer than risers. Total run is treads times tread run. The stringer cut line is the diagonal of total rise and total run.
The board must be longer than the cut line, so the lumber size allows one extra step diagonal for the top and bottom cuts, rounded up to the next standard length.
The throat is the board depth left under the deepest part of each notch. Thin throats split, so indoor stairs get a warning below 3 1/2". That figure is a rule of thumb, not code.
Flight, deck and stringer limits
Flight limits are from the 2021 IRC and IBC. Deck limits are from the AWC DCA 6 deck guide (2015 IRC edition, April 2018), which is a guide and not your local code, so check with your building department.
- Rise of one flight before a landing: 151 inches (12' 7") under the 2021 IRC (R311.7.3), 144 inches (12' 0") under the 2021 IBC (1011.8). Past that the calculator warns, but it doesn't lay out landings.
- Deck stair throat: at least 5". Even a 2x12 can fall short with a tall riser and deep tread.
- Deck cut stringer span: at most 6' 0" for any species. Past that, add a mid-span post or use solid stringers.
- Cut stringers, when no solid stringers are used: at least 3, at most 18" apart, center to center.
The shopping list counts 2x6 tread boards, cut to the stair width with no allowance for waste. Add a spare or two.
Code limits used
- 2021 IRC, residential: riser at most 7 3/4 inches (R311.7.5.1), tread at least 10 inches (R311.7.5.2). Risers in one flight may differ by no more than 3/8 inch, which equal risers meet. Treads under 11 inches need a nosing of 3/4 to 1 1/4 inches (R311.7.5.3).
- 2021 IBC, commercial: riser 4 to 7 inches, tread at least 11 inches (1011.5.2). Exception 3 to that section allows the 7 3/4 and 10 inch limits in Group R-3, inside Group R-2 dwelling units and in some Group U accessory buildings.
Tables for common rises
18 in · 24 in · 30 in · 36 in · 42 in · 48 in · 54 in · 60 in · 66 in · 72 in · 78 in · 84 in · 90 in · 96 in · 102 in · 108 in · 114 in · 120 in
Your local building department may use a different code edition or local amendments. Check with them before you build. This calculator is a planning aid, not engineering or code approval.