Enter the floor-to-floor rise and it returns riser height, tread count, total run and stringer length — with an IRC code check.
Riser count is set to the legal minimum automatically — increase it for a shallower, more comfortable stair.
Total rise is the vertical distance from finished floor to finished floor — not floor to ceiling, and it must include the thickness of the upper floor structure and both floor finishes. Getting this wrong is the classic stair-building error, and it is not recoverable once the stringers are cut.
Divide the total rise by the maximum allowed riser height and round up — that gives the minimum number of risers. Then divide the total rise by that count to get the actual riser height, which will be equal for every step. There is always one fewer tread than riser, because the last riser brings you level with the upper floor and no tread is needed there.
A floor-to-floor height of 108 inches (9 feet). Divided by the 7¾ inch maximum, that is 13.9, so you need 14 risers. Each is 108 ÷ 14 = 7.714 inches. That gives 13 treads, and at 10 inches each the total run is 130 inches — just under 11 feet of floor space, which surprises people planning a staircase into a tight hallway.
The angle works out at 37.6°, comfortably within the normal 30–37° range for residential stairs, though at the upper end.
| Requirement | Limit |
|---|---|
| Maximum riser height | 7¾ in (196 mm) |
| Minimum tread depth | 10 in (254 mm) |
| Minimum headroom | 6 ft 8 in (2032 mm) |
| Minimum clear width | 36 in (914 mm) |
| Maximum variation within a flight | ⅜ in (9.5 mm) |
That last one deserves emphasis. The tallest and shortest riser in a single flight may differ by no more than three eighths of an inch. The reason is that people climb stairs by pattern rather than by looking — after two or three steps the body has learned the rhythm and stops checking. A single riser that is half an inch off is a genuine trip hazard, and uneven risers are among the most commonly cited causes of stair falls.
Commercial stairs under the IBC are governed differently, generally 7 inch maximum rise and 11 inch minimum tread. Local codes vary and can be stricter than the model code — always confirm against the authority that will inspect the work.
Meeting code makes a stair legal. Three old rules make it feel right, and they broadly agree with each other:
2 × riser + tread = 24 to 25 inches. The most widely used, attributed to the 17th-century architect François Blondel. It reflects the mechanics of a walking stride, which shortens as it rises. Riser + tread = 17 to 18 inches. Riser × tread = 71 to 74 square inches.
In the worked example above, 2 × 7.714 + 10 = 25.4, slightly outside the comfortable band — the stair is a touch steep. Adding a fifteenth riser drops each to 7.2 inches and brings the rule to 24.4, at the cost of another 10 inches of floor run. That trade — steepness against floor space — is essentially the whole design problem of a staircase.
Stringer length is the hypotenuse of the total rise and total run, and you need a board somewhat longer than the calculated figure to allow for the cuts at each end. Measure and cut one stringer, then use it as a template for the others rather than measuring each independently — that is how variation creeps in.
Also remember that adding a floor finish after the stairs are built changes the top and bottom riser heights unless it is applied to the treads too. A quarter inch of tile at the bottom and nothing at the top puts you outside the ⅜ inch variation rule by itself.
Under the IRC, which governs residential construction in most of the United States, 7¾ inches. Commercial stairs under the IBC are limited to 7 inches with a minimum 11 inch tread. Local codes can be stricter than the model code, and some jurisdictions amend it — check with whoever will inspect the work rather than assuming the national figure applies.
Divide the total floor-to-floor rise by the maximum riser height and round up. For 108 inches at a 7¾ inch maximum, that is 13.9, so 14 risers, each at 108 ÷ 14 = 7.714 inches. You will have 13 treads — always one fewer than the riser count, because the top riser lands level with the upper floor and needs no tread.
Because people climb stairs by learned rhythm, not by looking at each step. After a couple of steps the body has fixed the spacing and stops checking, so a single riser that is noticeably different is a serious trip hazard. The IRC limits the variation within one flight to three eighths of an inch for this reason, and uneven risers are among the most commonly cited causes of stair falls.
More than most people expect. A standard 14-riser flight with 10 inch treads has a total run of 130 inches — nearly 11 feet — before you add landings or clearance at top and bottom. This is the constraint that drives most staircase design decisions, and it is why winders, spiral stairs and split flights with landings exist. Making a stair shallower for comfort always costs more floor.
Two times the riser height plus the tread depth should fall between 24 and 25 inches. It dates to the architect François Blondel in the 1670s and reflects how a walking stride naturally shortens as it climbs. It is a comfort guideline rather than a code requirement — a stair can meet code and still feel wrong underfoot, and this rule is usually why. Two related versions are riser plus tread of 17 to 18 inches, and riser times tread of 71 to 74.
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