How many stairs for a 9-foot ceiling, worked out

For a 9-foot ceiling the floor-to-floor rise is usually 118 to 122 inches once the upper floor’s joists, subfloor and finish are added. Under the International Residential Code a riser may be at most 7¾ inches, so divide the rise by 7.75 and round up: for 119 inches that is 15.35, so 16 risers of 7.44 inches (just under 7½), with 15 treads, because the top step is the upper floor. At the code’s minimum 10-inch tread, the stair runs 150 inches, 12 feet 6 inches, on plan. More risers make a gentler stair and a longer one. STAIR lists every riser count that fits your measured rise, with the tread each allows, checked against the IRC, the IBC and England’s Approved Document K.

Measure the rise, not the ceiling

The stair climbs from the finished floor downstairs to the finished floor upstairs. Starting from the ceiling height, add everything between the ceiling and the upper floor surface:

Measure it rather than adding it up if you can: at the stairwell opening, from the finished floor below to the top of the finished floor above. If the flooring is not down yet at either level, allow for its thickness at both, because a first or last riser that differs from the others is a trip hazard and the code limits the difference.

The IRC’s numbers

• Riser: at most 7¾ inches (R311.7.5.1), measured between the leading edges of adjacent treads. No minimum is set.
• Tread: at least 10 inches, nosing to nosing (R311.7.5.2).
• Consistency: within a flight, the tallest riser may exceed the shortest by no more than ⅜ inch, and the same for treads.
• Nosing: ¾ to 1¼ inches on stairs with solid risers, not required where treads are at least 11 inches.
• Headroom: at least 6 feet 8 inches, measured vertically from the line of the nosings (R311.7.2).

Commercial and other non-dwelling buildings come under the IBC, which is stricter: risers 4 to 7 inches and treads at least 11 inches (1011.5.2). Local amendments exist; your building department’s adopted code is the one that counts.

Working it through for 119 inches

Sixteen risers is the steepest stair the IRC allows here, and it is what most builders mean when they say "sixteen steps" for a 9-foot ceiling. Seventeen at 7 inches is noticeably easier on the knees and needs another tread’s worth of floor. A comfortable stair satisfies the old rule of thumb that twice the rise plus the tread is about 25 inches: 16 risers with 10-inch treads gives 24.9; 17 risers with 11-inch treads gives 25.0.

At a 10-foot ceiling the rise is about 131 inches and the minimum becomes 17 risers; at 8 feet, about 107 inches and 14 risers. Each foot of ceiling adds roughly one and a half risers.

The headroom check that catches people out

The stairwell opening in the upper floor has to be long enough that nobody climbing hits its edge. With the numbers above, the underside of the upper floor is at 108 inches; the nosing line must stay at least 80 inches below it, so the far edge of the opening must be above the point where the nosings are no more than 28 inches up, which is between the third and fourth treads, about 28 inches along from the bottom nosing. The top nosing is 150 inches along, so for a 16-riser stair with 10-inch treads the opening has to be roughly 122 inches long, a little over 10 feet, before allowing for the thickness of any trim. A shorter opening needs a steeper or turned stair.

In the calculator

STAIR takes the floor-to-floor rise in inches or millimetres and either the tread depth you want or the space the stair has to fit into, and lists every whole number of risers whose height is within the rules you choose (IRC, IBC or Approved Document K for England), with the tread each allows, the pitch, the total run and 2R + G. It shows its working, and says plainly what it does not check: headroom, handrails, guards, landings and winders. Building control, or your building department, is the authority. The measurements stay in the tab; the calculator is free to use and saving its working is part of OBSCURA Pro.