Calculate rise, run, number of steps, stringer length, and headroom for safe stairs.
A staircase is just a vertical rise divided into equal steps, and this tool does that division. You enter the floor-to-floor rise and the horizontal run the stair can occupy; it returns the number of risers, the riser height, the tread depth, the stringer length, and the headroom, and shows how the result sits against the usual residential limits: 7.75 in maximum riser, 10 in minimum tread, 80 in headroom. The step people get wrong most often is the off-by-one: the upper floor is the last tread, so a flight has one fewer tread than it has risers.
A concrete input and expected output from the current implementation.
Input
Total rise: 105 in. Total run: 140 in. Desired riser height: 7 in.
Expected output
15 risers at 7.0 in each; 14 treads at 10.0 in each; stringer length 175 in (14 ft 7 in).
105 in divided by 7 in is exactly 15, so the risers land at 7.0 in with no rounding, and the 140 in run split across 14 treads (15 risers minus the top step, which is the upper floor) gives exactly 10.0 in per tread. The stringer is the hypotenuse: the square root of 105 squared plus 140 squared is 175 in, the 3-4-5 triangle scaled by 35, which puts this flight inside the usual residential limits of a 7.75 in maximum riser and a 10 in minimum tread.
The tool divides the total rise by the riser height you want, rounds to a whole number of risers, then recomputes the riser height so every step is identical - residential rules allow only 3/8 in of variation between risers. The run is then split across the treads, and because the upper floor is the top step, the tread count is one less than the riser count.
The stringer is the diagonal member under the treads, and its length is the hypotenuse of the rise-and-run triangle: the square root of rise squared plus run squared. Headroom is measured vertically from the nosing line - the line through the front edges of the treads - to the underside of the floor above, and it varies along the flight, so an 80 in minimum has to hold at the tightest point.
Because the upper floor is the top step: a flight needs a riser for every step of vertical rise, but the final riser lands on the floor you are walking onto, which is already there. For a 105 in rise at 7 in risers that means 15 risers and 14 treads, and a stringer framed with 15 tread notches comes out one step too long.
The usual residential limits are 7.75 in maximum riser and 10 in minimum tread, and most builders land near 7 in risers with 10-11 in treads. The old Blondel rule of thumb, twice the riser plus the tread near 25 in, predicts comfort; a 6 in riser with a 12 in tread can pass the legal limits yet feel long and flat.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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