Stair calculator — rise, run and every mark on the stringer

Because the code says so and because your foot believes the first two steps. IRC R311.7.5.1 and R311.7.5.2 both put the limit at 3/8 inch: within one flight, the tallest riser may not exceed the shortest by more than that, and the same tolerance governs tread depth. Two or three steps in, a person stops looking and strides on the rhythm the flight has established, which is why an odd riser at the very top or the very bottom is the one that trips people rather than the one in the middle. It is also the easiest rule in the chapter to break by accident, since nothing about it shows up while you are cutting: the two ways it happens are a stringer that was not dropped by the tread thickness, and a floor finish laid after the stairs were framed.

Finished floor to finished floor, every time. A stair framed off subfloor heights is correct in every riser except one, and that one absorbs the entire thickness of whatever goes down later — 3/4 inch of hardwood on the lower floor shortens the bottom riser by 3/4 inch, and the same on the upper floor lengthens the top riser by the same amount. Two out of the 3/8 inch you are allowed, gone, from a decision made by the flooring installer months later. If the finishes are not chosen yet, frame to the thickest one you might use and shim, or enter the intended thickness in the two fields on this page so the as-built bottom and top risers are printed before you cut anything.

Fourteen. The top riser lands on the upper floor, and that floor is the last tread — so a flight always has one fewer tread than it has risers, and ordering fifteen treads for fifteen risers leaves you with a spare. The same off-by-one runs through the run: 14 treads at 10 inches is 11 feet 8 inches of floor, not 12 feet 6. It reverses only where the flight stops at a landing that is not the destination floor, in which case the landing is the tread and the counts are equal.

7 3/4 inches and 10 inches for a house, from IRC R311.7.5.1 and R311.7.5.2 in the 2021 edition. The riser is measured vertically between the leading edges of adjacent treads and the tread horizontally between the vertical planes of the foremost projections, which is to say nosing to nosing rather than across the board. Those are residential figures: a stair inside a commercial building answers to the IBC instead, where the numbers are 7 inches and 11 inches, and plenty of jurisdictions amend either book. Both limits are editable fields on this page for that reason, and the code your building department adopted is the one that governs, not the one quoted here.

Headroom plus the thickness of the floor above, multiplied by the run and divided by the rise. With 80 inches of headroom, a 12-inch floor build-up, a 10-inch tread and a 7 1/4-inch riser, that is (80 + 12) × 10 ÷ 7.25 = 126.9 inches, so the opening runs just under 10 feet 7 inches back from the edge of the upper floor. IRC R311.7.2 sets the 80 inches, measured vertically from the sloped line joining the nosings rather than from any one tread. The counter-intuitive half is that a gentler stair needs a longer hole: drop to a 7-inch riser and the opening grows to nearly 11 feet, because the flight descends away from the header more slowly.

Board depth minus rise times run over the hypotenuse — on a 2×12 at 7 1/4 by 10 inches, that is 11.25 − 5.87 = 5 3/8 inches of throat. The number matters because the throat, not the board, is the part actually carrying the flight, and it shrinks as the stair gets steeper: the same 2×12 notched 7 3/4 by 10 leaves 5 1/8, and a 2×10 takes exactly two inches off whatever a 2×12 gives, so the shallower cut leaves 3 3/8. The IRC prescribes no throat depth for a site-cut stringer, which surprises people — 5 inches is the figure most building departments and stringer suppliers print, some local amendments say 3 1/2, and a manufactured or engineered stringer carries its own rating. This page prints your number so you can hold it against whichever of those applies where you are building.

No, and it would not help anyway. The maximum riser is a limit, not a target, and going past it is the single most commonly cited stair violation — but the more useful point is that taller risers make the run longer, not shorter. Fewer risers means fewer treads, yes, but the flight has to reach the same floor: what actually shortens the run is a shallower tread, and that stops dead at the 10-inch minimum. Past that, the space is bought with geometry rather than dimensions: a landing that turns the flight through 90 or 180 degrees, or winders, which the code treats under their own rules measured at a walkline 12 inches in from the narrow edge. There is also a ceiling on the flight itself — R311.7.3 caps a single flight at 151 inches of rise between floors or landings, so a climb taller than 12 feet 7 inches has to be broken by a landing whether you want one or not.