Stair Calculator

Finished floor to finished floor, vertically — and 'finished' is where projects go wrong: if the lower slab gets ¾" flooring later, or the upper landing does, the rise changes by that amount, and since every riser must match, the error lands entirely on one step. Measure with flooring installed or add its planned thickness to the math now. For uneven ground at a deck stair's base, measure to where the bottom step will actually land, not to the ground at the deck post.

Because bodies learn a staircase's rhythm within two steps and then stop looking — a riser that's a half-inch different from its siblings is functionally invisible and genuinely dangerous, which is why the IRC allows no more than 3/8" variation between the tallest and shortest riser in a flight. It's the most-failed inspection item in stair building. The calculator divides your rise into perfectly equal risers; your job is transferring that number accurately to the stringer layout.

Under the IRC (adopted with local variations — always confirm locally): risers 7¾" maximum, treads 10" minimum depth, headroom 6'8" minimum, and the 3/8" uniformity rule. Nosing (the tread overhang) of ¾–1¼" is required when treads are under 11". Commercial (IBC) is stricter: 7" risers, 11" treads. These are ceilings and floors, not targets — the comfort formula usually lands you gentler than the maximums, and stairs at the legal limit feel like a ladder.

Twice the riser plus the tread should land between 24 and 25 inches — François Blondel's 1675 observation that a human stride shortens predictably as it climbs, still embedded in modern codes. Stairs that satisfy it feel effortless; violations announce themselves as that staircase where you always half-stumble. The calculator scores your combination live: too low means steep-and-cramped (deepen the tread), too high means a stretched, tiring gait — acceptable outdoors where strides run longer, annoying inside.