U-shaped stair calculator

Riser count and riser height are calculated the same way as any stair: total rise divided by target riser height, rounded, then divided back. A 108 in rise at a 7.25 in target gives 15 risers at 7.2 in each, same as a straight or L-shaped stair with the same rise.

Split those 15 risers between a lower flight and an upper flight, same as an L-shaped stair, but this time the landing turns the direction of travel 180 degrees instead of 90. A 7-riser lower flight has 6 treads and a run of 63 in; the remaining 8-riser upper flight has 7 treads and a run of 73.5 in.

After the landing, the upper flight runs directly back over (or alongside) the lower flight, which is what makes a U-shaped stair so compact in plan.

In a straight-through landing stair, the landing only needs to be as wide as one flight, because there's only ever one flight using it at a time in that direction. In a U-shaped stair, the landing has to accommodate the width of both flights side by side, plus any gap or wall between them.

For two 36 in wide flights with a small gap, that puts landing width around 72 in to 78 in, roughly double a single flight's width. This is the single most common mistake in laying out a U-shaped stair on paper: sizing the landing like a straight stair's landing and then finding it's too narrow once both flights are drawn in.

Landing depth (the dimension in the direction of travel) still follows the same commonly cited minimum of at least the stair's width, hedged since local codes vary.

Most U-shaped stairs split risers as close to even as the total allows, since the design usually needs both flights to look balanced in the same stairwell opening. With 15 total risers, a 7-and-8 split is about as even as you can get with an odd number.

An uneven split isn't wrong, but it's worth a sanity check: a landing set unusually high or low relative to the midpoint usually means the design is being driven by something else, like a floor opening or a door on an intermediate level, which is fine as long as it's intentional.

1. Calculate total risers and riser height for the full rise.
2. Split the risers as evenly as possible between the lower and upper flight.
3. Calculate each flight's tread count and run.
4. Size the landing depth to at least one flight's width.
5. Size the landing width to fit both flights side by side plus any gap.

A straight stair covering the same 108 in rise at the same 7.2 in riser height would need 14 treads at 10.5 in, a run of 147 in in one direction. A U-shaped stair covering the same rise needs a footprint roughly as long as the longer of its two flights plus the landing, around 73.5 + 72 = 145.5 in, but that length doubles back on itself instead of running in one direction.

The real savings shows up in usable floor plan: instead of needing a 147 in long hallway or room, you need roughly that same length in one direction but only one flight's width, because the second flight sits directly beside (or above) the first rather than extending the building's footprint further.

A U-shaped stair's landing carries more structural load than a single-flight landing, since it's wider and often supports a guardrail along the open well between the two flights. It's typically framed on its own posts or ledgers, independent of either flight's stringers, similar to a small floor section.

Because the open well between the two flights is a fall hazard, guardrails there need to meet the same commonly cited height and baluster spacing requirements as the rest of the stair, hedged since exact requirements vary by jurisdiction. Plan the rail posts into the landing framing before you build it, not after.