Stair headroom calculator

Headroom is measured vertically, not along the slope of the stair, from an imaginary line touching the leading edge (nosing) of every tread, up to the closest obstruction above: a header, ceiling, ductwork, or the underside of the floor framing on the level above.

Because that nosing line runs at the stair's angle while the ceiling above is flat, headroom is smallest right at the point where the stair first passes under the floor opening, and it gets larger as you walk further along the flight. That single tightest point is the one that matters.

A commonly cited minimum for residential stairs is 6 ft 8 in (80.5 in), measured at every point along the stair width and continuing for the full run of the flight, not just at one spot. Some jurisdictions and older code editions allow a reduced value directly under a header for a short distance.

Codes vary by location and by whether the stair is residential or commercial, so treat any number here as a starting point for planning, not a pass/fail answer. Always confirm the current requirement with your local building department before you frame an opening.

If you know the stair angle and the floor thickness above, you can work backward to find how long the floor opening needs to be for a target headroom. The relationship is approximately: opening length = (target headroom + floor thickness) ÷ tan(stair angle), measured from the point where the stair passes under the header.

That formula treats the floor structure above as a flat slab of a given thickness; in real framing the joist depth plus subfloor and finish flooring together make up that thickness, so measure the whole stack, not just the joist.

1. Find your stair angle from riser height and tread depth: angle = atan(riser ÷ tread).
2. Decide your target headroom, commonly 80.5 in (6 ft 8 in) as a starting reference.
3. Add the floor thickness above (joist depth + subfloor + finish floor) to the target headroom.
4. Divide that sum by tan(angle) to get the minimum opening length.
5. Check the result against the opening you actually have room to frame.

Take a stair with a 34.4° angle and a floor structure above that's 10 in thick (joists plus subfloor and flooring). Targeting the commonly cited 80.5 in headroom: (80.5 + 10) ÷ tan(34.4°) = 90.5 ÷ 0.685 ≈ 132.1 in of opening length.

That's about 11 ft along the run, measured from where the stair first passes under the header. If your existing rough opening is shorter than that, you either fall short of the target headroom or you need to change the stair's geometry.

Lengthening the floor opening is the most direct fix, since it lets the header move further from the point where the stair angle passes underneath. It's also often the least disruptive if there's framing room to spare in that direction.

A steeper stair (taller risers, shallower treads, within a comfortable and code-reasonable range) shortens the run needed to gain the same headroom, at the cost of a more aggressive climb. Dropping the header, boxing out the joists, or adding an intermediate landing that shifts the tight spot are the other common moves when the opening truly can't grow.

• Extend the rough opening further into the room above
• Steepen the stair slightly (more riser, less tread) if it stays reasonable to climb
• Notch or box the joists above the tightest point where structurally allowed
• Break the flight with a landing to move the pinch point

On a built stair, hook a tape to the nosing of the tread directly under the header, hold it plumb, and read straight up to the underside of the framing or finished ceiling. Repeat that at two or three treads near the tight spot, since drywall thickness and header depth both eat into the number in ways a drawing won't show.

If you're this calculator's inputs are for a stair you haven't built yet, use the opening-length formula above to check your framing plan before you cut a header, not after.