Stair Rise and Run
Risers and treads from your floor-to-floor height with the 8.25 in riser limit, the 10 in tread minimum, the total run, the slope, and the headroom and landing it needs.
A 106 in floor-to-floor at a 7.75 in riser is 14 risers with the upper floor as the last one, so 13 risers and 13 treads on the stringers. Tread depth is measured nose to nose, and the nosing overhang is part of the run, not the tread.
How the math works
- Riser count: risers = floor-to-floor height ÷ target riser, rounded, and the re-divided riser = total ÷ count so every riser is equal. The top riser is the floor itself, so the stringers carry count − 1 risers and count treads.
- Residential limits (IRC R311.7): riser 8.25 in max, tread 10 in min (9 in under the open-riser rules with a 4 in max gap), 36 in min width, 6 ft 8 in min headroom, 12 ft max rise per flight, 3.5 in max nosing overhang, and the winder rule of 10 in at the 24 in line.
- Commercial limits (IBC 1011): riser 7 in max and 4 in min, tread 11 in min, 36 in min width, 11 ft max rise per flight, 3/8 to 3/4 in nosing, and 2R + T between 60 and 66 in on the IBC spiral and winder provisions.
- Headroom: 80 in measured vertically from the leading edge of the tread (the nosing line) to the ceiling, soffit, or the bottom of the framing above the stair opening. A stair framed into a floor opening needs the opening length to clear the headroom line.
- Landings: a landing at the top and bottom at least as wide as the stair and 36 in deep in the direction of travel, level, and required where the rise exceeds 12 ft. A door swinging onto a landing needs 5 ft of landing in the IBC.
How many risers for a 106 inch stair?
14 risers at 7.57 in each with a 7.75 in target, and the stringers take 13 risers with 14 treads because the upper floor is the last riser. Rounding matters: 106 ÷ 14 = 7.571 in, and a stair with the last riser 3/4 in taller is the classic bad stair. A 10 ft ceiling with a 12 in floor assembly is 120 in floor to floor, which is 15 risers at 8 in, and 8 in is over the 8.25 in residential max only in older editions, so 16 risers at 7.5 in is the cleaner layout.
What is the comfortable stair formula?
2R + T between 24 and 25.5 in on the old rule, which is the same as 60 to 66 in in the IBC wording at 7 in risers and 11 in treads. A 7 in riser with an 11 in tread (2R+T = 25 in) is the stair everyone calls comfortable, and a 7.75 in riser with a 9 in tread is the stair everyone complains about.
How much headroom does a stair need?
80 in (6 ft 8 in) measured from the nosing line of the treads up the slope, not from the floor. A stair under a soffit at 6 ft 6 in fails by 2 in over the whole run, and fixing it means moving the stair opening 3 to 4 ft, not shaving the soffit.