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Stairs Calc

Stairwell opening

The hole framed in the upper floor that the staircase rises through.

The stairwell opening is the rough opening framed into the upper floor that the staircase climbs through. Its length must be long enough that headroom — the diagonal clearance from the nosing line — never drops below the 6 ft 8 in (2032 mm) minimum as the stair passes under the floor edge. Example: a deeper floor structure or a shallower stair angle forces a longer opening, because the headroom line clears the framing farther back. Get this wrong and tall users duck under the header. The calculators compute the minimum opening from your rise, run, and floor thickness so you can frame it before the stair is built, not after.

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Related terms

Headroom Vertical clearance measured plumb above the nosing line; minimum 6 ft 8 in (IRC). Stair angle (pitch) The slope of the flight in degrees; comfortable stairs sit around 30–37°. Nosing The lip of the tread that overhangs the riser below; ¾–1¼ in in the IRC. Total run The horizontal distance the whole flight covers, floor to floor.

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Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.

Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.

Last reviewed 2026-06-20 against IRC 2021/2024

Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.