The riser count is the only real decision
A stair layout looks like it has many variables and really has one. The total rise is fixed by the building. Once you pick how many risers to divide it into, everything else follows: the riser height is the rise divided by that count, the tread count is one fewer, the run is the treads times the tread depth, and the stringer is the hypotenuse over the lot. That is why this calculator asks for a target riser height rather than a riser height. You do not get to choose the riser height. You choose a riser count, and the height falls out of it.
The reason it works that way is R311.7.5.1. The greatest riser in a flight may exceed the smallest by no more than 3/8 inch, and the same tolerance applies to treads under R311.7.5.2. So you cannot make thirteen risers at 7 inches and one at 6 to use up the remainder — every riser has to be the rise divided evenly. Anyone who has felt a stair where the bottom step is short knows why the code cares; the commentary is blunt that variation beyond the tolerance interferes with safe travel.
Measure to finished floor, both ends
The most expensive mistake on this page is not arithmetic. It is measuring the total rise to a subfloor, cutting the stringers, and then having three-quarter-inch flooring go down at the top and not the bottom. That does not spread across the flight — it lands entirely on one riser, which is how a flight that was laid out correctly ends up outside the 3/8 inch tolerance. Work in finished dimensions from the start, even when the finish floor is months out and undecided. If it is genuinely unknown, cut the stringers last.
Legal is not the same as good
A flight at the exact minimums — 7¾ inch risers and 10 inch treads — passes inspection and is noticeably steep. That is why the result carries a rise-plus-run figure alongside the code verdict. Builders have aimed for one riser plus one tread landing between 17 and 18 inches for far longer than the IRC has existed. It has no legal force, a flight can satisfy it and still fail code, and it can fail the band while being entirely legal. It is a second opinion, not a requirement, and this page reports the two separately rather than blending them into a single pass/fail.
Tread depth is worth one more thought. The minimum is 10 inches, but at 11 inches R311.7.5.3 stops requiring a nosing at all. If you have the floor space, 11 inches removes a detail from the build, gives a more comfortable stair, and takes an inspection item off the list. It costs you one extra inch of run per tread — on a 13-tread flight, about 13 inches of floor.
What this layout does not cover
This is stair geometry. It does not size the stringer stock, check the remaining material behind the notches, or design the connection at the top and bottom — all structural questions that depend on span, species and support. It does not handle winders, spiral stairs, curved flights, or a landing partway up, each of which has its own subsection of R311.7. And it does not check headroom, because headroom is set by the floor opening above rather than by the flight: the code requires 6 ft 8 in measured from the sloped line adjoining the nosings, and that is the constraint that most often forces a stair to move rather than be re-cut.
Guards, handrail height and grip size, and the openings a 4-inch sphere may not pass through are all separate requirements in R311.7.8 and R312. Deck stairs carry the same R311.7 geometry, but the framing that supports them is sized separately — see deck joist spans for that side of the job.