Stair dimensions calculator

Riser height is the vertical distance between two consecutive tread surfaces. Tread depth is the horizontal distance you actually step on, not counting nosing overhang. Total run is the sum of all tread depths, and total rise is the sum of all riser heights.

Nosing is the tread's projection past the riser face below it, usually 3/4 to 1 1/4 in on closed-riser stairs. Stair width is measured horizontally between the walls or guards the stair runs between, usually above handrail height.

Headroom is the vertical clearance from the nosing line of each step straight up to the ceiling or framing above. All of these come from the same rise, riser and tread inputs, which is why one calculator can return them together.

Residential and commercial stairs follow different reference standards because the expected use is different — a house stair is used by people familiar with it, a commercial stair by the general public, sometimes under emergency evacuation conditions.

Commonly cited IRC reference values for residential stairs: maximum 7 3/4 in riser, minimum 10 in tread, maximum 3/8 in variation between risers in a flight, 6 ft 8 in minimum headroom, 36 in minimum width.

Commonly cited IBC reference values for most commercial stairs: maximum 7 in riser, minimum 11 in tread — gentler than residential minimums, reflecting stricter public-safety requirements. Always confirm both with your local authority having jurisdiction, since amendments are common.

Riser height comes from dividing total rise by the rounded riser count: riserHeight = totalRise / round(totalRise / targetRiser).

Tread count follows from riser count assuming a flush top step: treads = risers − 1. Total run is treads × tread depth.

Stringer length uses the Pythagorean theorem on total rise and total run: stringerLength = √(rise² + run²). Angle of the stair is atan(riserHeight / treadDepth), converted to degrees.

Headroom is checked by comparing your rough opening length against where each nosing line crosses under the header, using the stair's rise and run slope.

Handrail height is commonly cited in the 34–38 in range, measured vertically from the nosing line to the top of the rail. Guard height (the barrier along an open side of a stair or landing) is commonly cited around 36 in residentially, higher in many commercial settings.

Baluster spacing along a guard is commonly limited so a 4 in sphere can't pass through, to keep small children from slipping between rails — again, confirm the exact figure locally.

Nosing is measured from the riser face to the leading edge of the tread above it. Too little nosing (under 3/4 in) can feel abrupt; too much (over about 1 1/4 in) on a tight radius can catch a heel.

Stair width is typically measured between finished wall surfaces or the inside face of guards, above the handrail height where rails may project a couple inches into the stairway on one or both sides.