Stair risers = total rise ÷ 7½ in, rounded to a whole number, and stair treads = risers − 1. A 9 ft (108 in) rise takes 14 risers of 7 ¹¹⁄₁₆ in and 13 treads; on a 36 in wide stair that is 13 tread boards and 14 riser boards to cut. This stair riser calculator and tread calculator work from the same rise, so the counts, the boards and the code check always agree.
How stair treads and risers are calculated
The riser count comes first. The rise is divided by your target riser (7½ in by default) and rounded, and the count is raised if that would leave a riser above the code limit of 7¾ in (IRC R311.7.5.1). Every riser is the rise divided by that count, so all of them match. A straight flight has one tread fewer than it has risers, because the floor at the top is the last step.
Risers = total rise ÷ target riser, rounded (and at least total rise ÷ 7¾ in). Treads = risers − 1. Tread board depth = tread depth + nosing. Boards per tread = ⌈(depth + gap) ÷ (board width + gap)⌉.
For the default 9 ft rise: 108 ÷ 7.5 = 14.4, so 14 risers, each 108 ÷ 14 = 7.71 in. There are 13 treads of 10 in, and with the 1 in nosing each tread board is 11 in deep. A 5/4×12 board is 11¼ in wide (NIST PS 20), so one board makes one tread and 13 tread boards are cut. On a 36 in stair the cuts pack three to a 10 ft board, four to a 12 ft board and five to a 16 ft board, so the cheapest buy is three 16 ft boards. Riser boards work out the same way: 14 cuts from three 16 ft boards.
Stair riser boards: which board and how much to rip
A riser board has to be at least as tall as the riser. A 1×8 is only 7¼ in wide, shorter than a 7 ¹¹⁄₁₆ in riser, so the calculator picks a 1×10 (9¼ in) and tells you to rip 1 ⁹⁄₁₆ in off it. Trim the bottom riser by the thickness of the tread so it does not stand proud once the treads are on. Deck stairs with open risers need no riser boards, but any opening between treads must stay under 4 in where the stair is more than 30 in above grade (R311.7.5.1).
Stair treads for a deck: 2×6 boards and a gap
Deck stairs usually take two 2×6 boards a tread. Two boards of 5½ in with a ⅛ in gap (the spacing the American Wood Council's DCA 6 gives for decking) cover 11⅛ in, enough for a 10 in tread with a 1 in nosing. A 3 ft rise gives 5 risers of 7 ³⁄₁₆ in and 4 treads, so 8 boards, which come from three 10 ft boards. Pick "Deck stair" and the calculator switches to 2×6 boards and drops the riser boards.
Stair treads and risers by height
With a 7½ in target riser and 10 in treads:
| Total rise | Risers | Riser height | Treads | Total run |
|---|---|---|---|---|
| 2 ft 6 in | 4 | 7 ½ in | 3 | 2 ft 6 in |
| 3 ft | 5 | 7 ³⁄₁₆ in | 4 | 3 ft 4 in |
| 4 ft | 7 | 6 ⅞ in | 6 | 5 ft |
| 6 ft | 10 | 7 ³⁄₁₆ in | 9 | 7 ft 6 in |
| 8 ft | 13 | 7 ⅜ in | 12 | 10 ft |
| 9 ft | 14 | 7 ¹¹⁄₁₆ in | 13 | 10 ft 10 in |
| 10 ft | 16 | 7 ½ in | 15 | 12 ft 6 in |
Stair codes in the US and Canada
The IRC limits risers to 7¾ in and treads to 10 in minimum, and asks for a nosing of ¾ to 1¼ in unless the tread is at least 11 in deep (R311.7.5). The greatest riser in a flight may not exceed the smallest by more than ⅜ in, which is why the rise is divided equally. The British Columbia Building Code allows private stairs a rise of 125 to 200 mm and a run of 255 to 355 mm, with a tread no more than 25 mm deeper than the run (Tables 9.8.4.1 and 9.8.4.2). Choose the rulebook in the calculator: in metric, a 2.74 m rise with 255 mm treads gives 14 risers of 195.7 mm. Boards and nosings are entered in millimeters.
Next steps for the stair
The boards sit on stringers: lay them out with the stair stringer calculator, and check headroom and the stringer angle with the stair calculator. Long flights need a stair landing. Cover the treads afterwards with the stair runner calculator, and space the balusters for the rail with the baluster spacing calculator. For deck stairs, the deck board calculator counts the boards on the deck itself. Browse everything on the decks and stairs hub.