How a stair is measured
Four measurements, in the order every page here asks for them: the total rise floor to floor as the floors stand today, the clear width, the tread measured nosing to nosing, and the tallest riser you will allow — the code's 7¾ inches unless your town says otherwise. Flooring still to go down belongs in the two flooring rows under Assumptions, not in the rise.
The riser count is the rise divided by that riser, rounded up. A rise of 105 inches takes 14 risers of 7½ inches; the top floor is the last step, so there are 13 treads and 10 feet 10 inches of run. Rounded to the nearest, 101 inches becomes 13 risers of 7.77 inches, taller than the code allows; rounded up it is 14 of 7 3/16.
Two flights are two parts on one list. A rise over 12 feet 7 inches has to be split by a landing, and the split need not be even: on the landing page, 156 inches split as 90 and 66 takes 21 risers where 78 and 78 takes 22, because each flight rounds its own count up.
Three places the obvious arithmetic is wrong
The bottom riser. Every tread lies on the stringer except at the floor, so the bottom riser is cut shorter by the tread's thickness — to 6 inches on the 105 inch stair — and every finished step comes out at 7½. Cut every riser the same and the first step is 9 inches and the last 6, where the code allows ⅜ inch of difference in a flight.
The board. That stringer is 13 feet 6½ inches along its notches, but the board also has to hold the plumb cut at the top and the seat at the foot: 13 feet 10⅛ inches, so a 14 foot 2×12. A calculator that counts the point in front of the first riser — the piece that is cut off and thrown away — asks for 16 feet. Under the notches a 2×12 keeps 5¼ inches and a 2×10 only 3¼, where the deck guide asks for 5.
The stringer count. It is the width divided by the spacing, rounded up, plus one. A 40 inch stair at 18 inches takes 4; counting grid lines gives 3 and leaves 20 inches between them. Composite treads want stringers closer, and at 12 inches a 48 inch deck stair takes 5.
Where the numbers come from
The limits are read in the code text of two editions: IRC 2021 R311.7 and R312.1, and IRC 2024, which moved the stairway to R318.7 and the guard to R321.1 with every figure kept. Spiral stairs have one clause, R311.7.10.1, renumbered R318.7.11.1. Each check cites the clause of the edition you pick.
Every limit is a box, because states amend them, and the riser and the tread are the two they change most: some adoptions of IRC 2021 allow a taller riser on a shorter tread than the model code. Type your town's figure in and the check is marked "Local rule used".
A deck stair adds the American Wood Council's DCA 6-15: 2×12 stringers at least, three of them or more and no more than 18 inches apart under wood treads, a cut stringer spanning 6 feet at most, a landing past 12 feet of rise. The dressed width of a 2×12 is NIST PS 20-20; composite spacings are the tread makers' installation guides, by page. The 1 inch nosing, the 1½ inch tread and the 25 inch comfortable step are conventions. The spiral walkline, 12 inches out from the column, is our reading of a winder clause, labeled an estimate, and every price is an estimate.
The checks that never pass on their own
The guard is never marked "Meets": the code wants one along an open side more than 30 inches above the floor or ground below, and the page knows the rise between two floors, not the ground beside the stair. From 4 risers a handrail is required and the page cannot see one, so that check is marked "Not checked". Headroom meets only when every flight is measured under its opening; on the headroom page, the 105 inch stair under a 120 inch opening in a 10 inch floor clears exactly 6 feet 8 inches.
Spiral headroom never passes either: the page knows the tread a full turn up, not how far the kit's landing reaches, so under 6 feet 6 inches it fails and over it the kit's drawing decides.
What these pages do not do
They do not split a flight for you: past 12 feet 7 inches the check fails and says to add a landing; you place it. They do not frame the landing or check its 36 inch depth, since no box measures one.
They do not work a deck stair down sloping ground. They count treads, not the 2×6 or 5/4 boards a tread is made of, and no hangers, brackets or concrete pad at the foot. They do not check open risers against the code's sphere, or print a full-size stringer template.
The railing page counts wood balusters, posts and rail, not an aluminum or vinyl kit, and cannot tell whether a post holds the load a guard must. The spiral page lists no lumber: the kit is the purchase.