Stairwell Opening Calculator: How Long Should the Opening Be?

The stairwell opening formula

Climbing a stair raises your head by one riser for every tread. The floor above is a fixed ceiling, so the opening must be long enough that, at the point where the floor framing starts, the sloped line through the tread nosings is already far enough below it. That vertical distance is the headroom you need plus the thickness of the floor itself.

Clearance below the floor edge = headroom + floor thickness
Opening length                 = clearance × tread run ÷ riser height
Treads along the opening       = ⌈clearance ÷ riser height⌉

Use inches throughout. The first line is the vertical drop from the top of the finished upper floor to the nosing line. The second line turns that drop into horizontal distance, because the nosing line falls one riser height for every tread run. The third line rounds up to whole treads for when you want the opening to end on a layout line. The riser height is the actual equal riser (total rise divided by the number of risers), not the maximum you started with.

Worked example: a 105 inch rise

A flight rises 105 in from finished floor to finished floor. The floor between the two levels is 2×10 joists with a subfloor, finish flooring and a ceiling below, 11 1/4 in in all. The design uses 10 in treads and keeps each riser at or below 7 3/4 in.

Risers            ⌈105 ÷ 7.75⌉ = 14, so each riser is 105 ÷ 14 = 7.50 in
Clearance         80 + 11.25 = 91.25 in
Opening length    91.25 × 10 ÷ 7.50 = 121.67 in = 10′ 1 11/16″
On whole treads   ⌈91.25 ÷ 7.50⌉ = 13 treads, 13 × 10 = 130 in = 10′ 10″
Check             13 × 7.50 − 11.25 = 86.25 in of headroom at that edge

The flight has 13 treads, so its total run is 130 in. An opening of about 10′ 1 11/16″ leaves only the last 8.3 in of the flight, at the bottom, running under the floor. That is why the opening for a full flight is almost as long as the flight itself.

To find the risers, treads and total run for another total rise, use the stair rise and run chart.

Opening length by floor thickness and riser height

Minimum opening length for 80 in of headroom and 10 in treads. A taller riser needs a shorter opening, and a thicker floor needs a longer one.

These are minimums measured along the stair. Add whatever your framing, trimmer and header details need, and check the result against the drawn stair section.

What counts as floor thickness

Floor thickness is the distance from the top of the finished upper floor to the lowest surface that someone on the stair could hit, which is usually the ceiling finish below. Count every layer in that stack.

Add the layers that apply to your floor. A 2×10 floor with a 3/4 in subfloor, 3/4 in hardwood and 1/2 in ceiling drywall is 9 1/4 + 3/4 + 3/4 + 1/2 = 11 1/4 in, the value used above. Engineered joists, trusses, soffits and ducts change the total, so measure your own assembly.

How headroom is measured

The International Residential Code asks for at least 6 ft 8 in of headroom in all parts of a stairway, measured vertically from the sloped line adjoining the tread nosings (R311.7.2). The International Building Code uses 80 in measured the same way (1011.3). The calculator defaults to 80 in, which is the same distance as 6 ft 8 in. Some editions and some jurisdictions add exceptions, and spiral stairs have their own provisions, so confirm the numbers that apply to your project.

One exception in the residential code lets a floor opening project a few inches into the required headroom where the treads at the side of the flight extend under its edge. It is not built into the calculator because whether you may use it depends on your edition and on how the stair is framed. Ask your inspector before relying on it.

Headroom is measured to the lowest obstruction, not to the joists. Ceiling drywall, soffits, ducts, beams and light fixtures all count. If a beam crosses the stair below the opening, the clearance under that beam sets the opening length, not the floor.

The shortest opening a code-limit stair allows

When space is tight, a steeper stair gives a shorter opening. The residential limits are a 7 3/4 in maximum riser and a 10 in minimum tread. A gentler stair with 7 in risers and 11 in treads is more comfortable but needs a longer opening.

Moving toward the steepest legal stair saves opening length but costs comfort. The 2R + T guideline on the stair calculator page shows how to weigh the two. A landing and a turn also shorten the straight opening, because the flight no longer passes under one long floor edge.

Mistakes that produce a short opening

• Mixing up the opening with the covered part of the stair. (Total rise − headroom) ÷ riser × run gives the length of stair that could sit under a floor with no thickness at all. For the example above that is 33.3 in, while the opening you frame is about 10′ 1 11/16″. Using the first number as the opening gives about 27% of the right length.
• Leaving out the floor. Headroom measured to the underside of the joists ignores the subfloor, finish flooring and ceiling finish that sit above and below them.
• Measuring headroom from the lower floor instead of the nosing line. Headroom is measured up from the sloped nosing line, which meets the upper floor at the top of the stair and drops one riser for every tread going down. Measuring up from the lower floor overstates the clearance.
• Using the maximum riser instead of the actual one. The actual equal riser is total rise ÷ number of risers. It is usually a little below the maximum, so the opening comes out slightly longer than a calculation on the maximum riser.
• Forgetting the foot of the stair. If the floor is thick and the rise is small, the ceiling at the bottom of the stair can be lower than the headroom you need. See the low-rise example below.

When the floor is too thick for the rise

A flight rises only 84 in to a floor that is 11 1/4 in thick. The ceiling at the foot of the stair is 84 − 11.25 = 72.75 in above the lower floor, which is less than 80 in. Lengthening the opening along the flight does not fix that, because any floor left over the foot of the stair has only that clear height under it. The calculator flags this on its "Under the floor at the foot" line.

The remedies are to extend the opening over the foot of the stair as well, to reduce the floor thickness, or to move the stair so its foot is under a higher ceiling. A taller flight gives a similar length: a 108 in rise under a 13 1/4 in floor needs about 10′ 0 7/8″, with 9.1 in of the flight at the bottom under the floor.

Opening width and where to measure

The opening needs a clear stair width above the handrails of at least 36 in in the residential code (R311.7.1), with handrails permitted to project a limited distance into that width. The framed opening is wider than that clear width because it also has to hold the finished wall surfaces, any stringer or skirt board, and the trimmers. Take the framed width from the plan, not from the stair alone.

For length, measure along the stair from the face of the top riser, where the finished upper floor ends, back toward the bottom. A top landing, a nosing that projects past the riser face and a header all move that starting line, so mark it on the plan before you transfer it to the floor.

Check it before you cut

A full-size check costs minutes and a miss costs a floor. Snap a line on the lower floor along the stair centerline and mark each riser position with its height. Draw the nosing line from the first tread nosing to the upper floor edge. At the planned edge of the opening, hold a plumb line from the lowest surface of the floor above down to the nosing line and measure that vertical distance. It must be at least the headroom you need. If it falls short, lengthen the opening, or revisit the riser, the tread run or the floor depth, before any framing is cut.

Sources

Stair headroom is covered in the International Residential Code, Section R311.7.2, and in the International Building Code, Section 1011.3, both published by the International Code Council. Your jurisdiction adopts a particular edition and may amend it, so use the text your building department enforces.

Frequently asked questions