Stair Headroom Calculator
How long does the hole in the floor above a stair need to be? Enter the stair and the floor depth to get the shortest opening that keeps the headroom you need, or check the headroom an opening gives.
Worked example: edit any value
Headroom
Angle 36.3° Total run 11 ft 4 1/2 in
A 10 ft 0 in opening leaves only 76 3/16 in at its edge, 3 13/16 in short. Lengthen the opening to at least 10 ft 5 3/16 in, or use a steeper layout if your limits allow.
How this result was calculated
- The stair
14 risers of 7.7143 in, 13 treads, going 10.5, nosing 1 - Minimum opening: where the nosing line is the required headroom below the floor structure
L = (H + d) × g ÷ h + nosing = (80 + 11.25) × 10.5 ÷ 7.7143 + 1 = 125.2014 inMeasured horizontally from the face of the top riser (the floor edge) back down the stair. Headroom is measured vertically from the line of the nosings, as in IRC 2024 R318.7.2. - Headroom for your opening
(L − nosing) × h ÷ g − d = (120 − 1) × 7.7143 ÷ 10.5 − 11.25 = 76.1786 in
All values (table)
| Item | Value | Note |
|---|---|---|
| Minimum opening length | 10 ft 5 3/16 in | From the top riser face, horizontally |
| Headroom at your opening | ≈ 76 3/16 in | At the opening edge |
| Required headroom | 80 in | IRC 2024 R318.7.2: 6 ft 8 in |
| Floor depth | 11 1/4 in | Upper finished floor to ceiling below |
| Risers / treads | 14 / 13 | |
| Riser height | ≈ 7 11/16 in | 7.7143 in |
| Total run | 11 ft 4 1/2 in |
Where headroom runs out
Headroom is the clear height above the line of the nosings. On a stair that climbs up through a floor opening, it is smallest right under the edge of the opening, where the floor structure starts. Further down the stair the ceiling is still overhead but the nosing line is lower, so there is more room; past the edge there is no structure at all. So the question is: how far back from the top of the stair must the opening edge be?
The IRC sets 6 ft 8 in of headroom for house stairs, measured vertically from the sloped line of the nosings (2024 R318.7.2; R311.7.2 in 2021). It allows the opening edge to project up to 4 3/4 in into that space where the tread nosings at the side of the flight extend under it. This calculator does not apply that exception; it gives the full-headroom opening.
Worked example
A 108 in rise with 14 risers of 7.714 in, 10 1/2 in goings and 1 in nosings, under a floor that is 11 1/4 in deep from the finished surface to the ceiling below (2x10 joists, subfloor, flooring and drywall):
- Minimum opening = (80 + 11.25) × 10.5 ÷ 7.714 + 1 = 125.2 in (about 10 ft 5 3/16 in), measured horizontally from the face of the top riser.
- A 120 in (10 ft) opening leaves about 76 3/16 in of headroom at its edge: 3 13/16 in short.
- If a tread is built level with the upper floor (flush top), the stair moves one going further under the floor and the opening has to grow to about 135.7 in.
These cases are automated test fixtures (headroom H1 and H2, with a property test that the formula is exact).
Measuring the floor depth
Floor depth is from the upper finished floor down to the lowest thing at the opening edge: drywall, a beam, a header. Include flooring that is not down yet. A deeper header at the opening edge needs a longer opening, so it is worth measuring there, not in the middle of the room.
Questions people ask
What is the minimum headroom required by code?
Under the IRC for houses, 6 ft 8 in (80 in), measured vertically from the sloped line of the tread nosings, or from the floor of a landing (2024 R318.7.2). Spiral stairs need 6 ft 6 in (R318.7.11.1). Commercial stairs under the IBC also need 80 in (1011.3). Your local adopted code decides.
Related tools and guides
- Stair Calculator Risers, treads, total run, angle and a to-scale side view from one height.
- Stair Landing Calculator Risers per flight, landing height and footprint, straight or L-shaped.
- Stair code requirements Which rulebook covers which stair, with edition-correct section numbers.
- Stair Angle Calculator Angle, grade and pitch from rise and run, or the run needed for a target angle.