How the rise & run calculator works
The method
Count risers first: ceil(total rise ÷ 7.75) — then equalize every riser to the exact fraction (an uneven riser is the first thing inspectors measure). Tread follows from the comfort rule 2×riser + tread ≈ 25 in (Blondel's 17th-century formula that still predicts comfort). Total run = tread × (risers − 1) — the top step lands on the upper floor, so it consumes no tread.
Reading the angle
37–38° is the residential sweet spot; IRC allows 20.8–42.8° implicitly through the rise/run caps. Steeper stairs eat less floor and more knees; shallower stairs eat the room — the calculator shows where you land.
The variance rule
Every riser and tread must match within 3/8 inch of its siblings — the leading trip-hazard code in staircases. The practical discipline: cut ONE layout stringer as the template, clamp it to each blank, and trace. Never re-measure per stringer; copy the template.