How to use the stairwell opening calculator
A stair needs a hole in the floor above it, and the size of that hole is set by headroom. Make it too short and someone climbing the stair hits their head on the edge of the floor; make it too long and you lose floor area, add trimming and spend more on framing or structure. This calculator works out the shortest opening that keeps the required headroom along the whole flight. It's a companion to the stair rise and run calculator, which sizes the risers and treads themselves.
- Pick a code preset. IRC, IBC and UK Part K fill in the maximum riser, minimum tread and required headroom. Choose Custom to type your own limits.
- Enter the floor-to-floor height, finished floor to finished floor, and the floor thickness at the opening: the full depth from the top of the floor above to the underside of its ceiling, header or slab soffit.
- Set the tread depth you plan to use and the stair width including stringers and wall linings.
- Read the results: number of risers, actual riser height, total run, minimum opening length, how many treads sit under the opening and the opening area.
Stairwell opening formula
Risers n = roundup(H ÷ Rmax), riser R = H ÷ n
Total run = (n − 1) × T
Minimum opening length L = (headroom + floor thickness) × T ÷ R
Opening area = L × stair width
Here H is the floor-to-floor height and T the tread depth. The nosing line climbs R for every T it moves forward, so to clear the headroom plus the depth of the floor overhead it has to travel (headroom + thickness) × T ÷ R horizontally before the floor above can start. The opening runs from that point to the top riser. If L comes out longer than the total run, the whole stair sits under the opening and the floor-to-floor height is too low for the floor build-up, which usually means a thinner floor at the edge or a steeper stair.
| Code | Max riser | Min tread | Headroom |
|---|---|---|---|
| IRC R311.7 (dwellings) | 7 3/4 in | 10 in | 6 ft 8 in (80 in) |
| IBC 1011 (most other buildings) | 7 in | 11 in | 80 in |
| UK Approved Document K, private stair | 220 mm | 220 mm | 2,000 mm (1,900 mm loft) |
Worked examples
A US house on 9 ft floors
Floor-to-floor 108 in, IRC limits, 10 in treads. That needs 14 risers of 7.71 in and 13 treads, a total run of 130 in. The floor at the opening is 2x10 joists with subfloor, flooring and drywall, about 11 1/4 in deep. The minimum opening is (80 + 11.25) × 10 ÷ 7.71 = 118.3 in, or 9 ft 10 1/4 in. Allow 3/4 in more for the nosing and frame the opening at about 10 ft.
An office stair under the IBC
Floor-to-floor 156 in (13 ft), 7 in maximum riser, 11 in treads, a 14 in steel and concrete floor. That gives 23 risers of 6.78 in and a run of 242 in. The opening must be (80 + 14) × 11 ÷ 6.78 = 152.4 in, roughly 12 ft 8 1/2 in, much longer than the house stair because the IBC stair is shallower.
A UK house to Part K
Floor-to-floor 2,800 mm, 220 mm maximum rise, 250 mm going, 250 mm floor depth. That is 13 risers of 215 mm and a 3,000 mm run, with 2R + G = 681 mm, inside the 550 to 700 mm range. The opening is (2,000 + 250) × 250 ÷ 215.4 = 2,612 mm. With a 900 mm stair the opening is about 2.35 m².
Who this calculator helps
- Architects and architectural technologists setting out stair cores and floor openings in early plans and sections.
- Structural engineers sizing trimmers, headers and slab openings around the stair.
- Builders and framers checking opening sizes before cutting joists on site.
- Loft conversion and renovation designers fitting a new stair into an existing floor.
- Interior designers planning a feature stair in an open-plan space.
- Students learning how stair geometry and headroom interact.
Practical tips
- Check every overhead obstruction, not just the floor edge. A beam, duct or pendant light inside the opening can break headroom further up the flight.
- Use the real depth of the header or trimmer at the opening edge, which is often deeper than the typical joists.
- A turn at the top, with a landing or winders, shortens the opening. Split the rise and run each flight separately.
- Convert site measurements quickly with the feet and inches converter, and check the joists that trim the opening with the joist span calculator.
- For the stair's angle and pitch on drawings, the slope calculator converts rise and run to degrees.
Once the stair works on plan, show the client what it will look like: sketch it in context and turn it into a photoreal view with Sketch to Render.
