Measuring for a floating stair before you calculate
A floating stair is judged on how thin and even the treads look, which means the one measurement most people skip — stair width — matters as much as total rise here.
Find your total rise
Hold a tape measure or a straight board plumb from the finished floor you're leaving to the finished floor you're arriving at. Include finished flooring thickness on both ends, not just the sub-floor.
Decide your stair width up front
Unlike a boxed stair, width isn't just a code minimum here — it directly sets how far each tread has to cantilever unsupported. Measure the opening you're building into, then decide how much of that you actually want the tread to span.
Pick a support system before you pick a tread material
A center mono-stringer and a wall-mounted cantilever put the unsupported overhang in different places (side-to-side vs front-to-back), and that changes what tread thickness you can get away with. Decide this before ordering material.
Read the compliance and advisory chips
Green and red chips are hard IRC limits — riser, tread, angle, landings. Amber chips are planning-stage advisories on open-riser gap and cantilever length that need a local code check or an engineer's sign-off, not a pass/fail from this page.
The five formulas behind every result
Same rise-and-run math as any staircase, plus the two numbers that actually make a stair "float": open riser gap and cantilever length.
Step Count
Total rise divided by your target riser height, rounded to the nearest whole step — you can't build a fractional stair.
Actual Riser Height
The rounding above means your real riser is rarely the exact target. This is the number every riser gets cut to, uniformly.
Tread Depth (2R+T rule)
The standard comfort formula, floored at the 10" IRC minimum, same as a conventional stair — a floating stair still has to be comfortable to walk.
Open Riser Gap
The vertical clear opening between the top of one tread and the underside of the next. This is the number a code reviewer measures with a sphere gauge, and it's usually bigger than people expect once a thick steel-core tread is subtracted from the riser.
Cantilever Length
Wall: C = Tread Depth − Engagement
How far each tread projects past its support with nothing underneath it. Center mono-stringers cantilever side-to-side; wall-mounted treads cantilever front-to-back. [VERIFY: confirm engagement/beam-width defaults with your fabricator or engineer.]
Walking through a real 126" rise, 40" wide floating stair
Same formulas, filled in step by step, for a great-room floating stair climbing 10'-6" (126") on a center mono-stringer — the default this calculator loads with.
Step count and riser
126" total rise, targeting a 7.5" riser: 126 ÷ 7.5 = 16.8, which rounds to 17 steps. Re-dividing gives the actual riser: 126 ÷ 17 = 7.41" per riser — the number every riser gets cut to.
Tread depth and angle
2R+T rule: 25 − (2 × 7.41) = 10.18", comfortably above the 10" minimum this time, so no flooring kicks in. The angle works out to atan(7.41 ÷ 10.18) ≈ 36.1° — inside the 30–37° comfort range, but only just.
Open riser gap
With a 2.25" steel-core tread: 7.41 − 2.25 = 5.16" of open gap between treads. That's over the commonly-cited 4" sphere limit [VERIFY], and since total rise is well past 30", the usual small-stair exception to that rule doesn't apply — this design needs a code check on the gap before it's finalized, even though riser, tread and angle all pass individually.
Cantilever — the number that actually drives the budget
At 40" wide on an 8"-wide center beam: (40 − 8) ÷ 2 = 16" of cantilever per side. That's beyond the roughly 12–14" many fabricators will do without a stamped structural review [VERIFY], which is a direct consequence of choosing a wide, dramatic-looking tread on a narrow center beam — the fix is a wider beam, a thicker tread, or an engineer's sign-off, not a redesign of the stair's rise and run.
This is the pattern worth remembering: a floating stair can pass every riser, tread and angle check that a conventional stair would, and still need engineering just because of the width and gap decisions that make it look "floating" in the first place.
Support systems and tread materials compared
"Floating" describes what you see, not what's holding the stair up — every option below hides real structure somewhere. Figures marked [VERIFY] are planning-stage rules of thumb; get final numbers from your fabricator or engineer.
| Support Type | Visible Structure | Typical Cantilever* | Relative Cost | Best For |
|---|---|---|---|---|
| Center Mono-Stringer | None (hidden below) | 10–14" per side [VERIFY] | $$$ | Open-tread look with treads fully clear on both sides. |
| Wall-Mounted Cantilever | None (embedded) | Up to ~36" depth [VERIFY] | $$ | One side against a masonry or engineered wall; cheapest true floating look. |
| Dual Steel Stringers (clad) | Thin cladding only | Minimal — nearly full support | $ | Floating aesthetic on a budget, least structural risk. |
| Tread Material | Typical Thickness* | Relative Weight | Relative Cost | Notes |
|---|---|---|---|---|
| Steel-Core, Wood-Clad | 2"–2.5" [VERIFY] | Heavy | $$$ | Hidden steel flat bar or plate does the structural work; wood is a wrap, not the load path. |
| Solid Hardwood Slab | 2"–3" [VERIFY] | Heavy | $$$$ | No hidden steel — the wood itself has to carry the cantilever, so thickness is non-negotiable. |
| Solid Steel Plate | 0.375"–0.75" [VERIFY] | Very heavy per volume | $$ | Thinnest possible profile for a given span; usually the choice when the tread itself must look ultra-thin. |
Want the steel sizing worked out in detail? Use the Steel Stair Calculator.
IRC residential stair code, including open-riser rules
A floating stair has to pass the same riser and tread checks as any stair, plus one that only matters when there's no riser board to close the gap.
| Element | Residential (IRC) | Commercial (IBC) |
|---|---|---|
| Max riser height | 7.75" | 7" |
| Min tread depth | 10" | 11" |
| Riser variance across a flight | 3/8" | 3/8" |
| Min headroom | 6'8" | 6'8"–7'6" (varies) |
| Min clear width | 36" | 44"+ (varies) |
| Open riser opening (sphere rule) | < 4" sphere [VERIFY] | < 4" sphere [VERIFY] |
| Open riser exception | Total rise ≤ 30" [VERIFY] | Rarely applies |
| Guard height, open sides | 36" [VERIFY] | 42" [VERIFY] |
Reference only — always confirm against your local jurisdiction's adopted code before building. The riser, tread, headroom and width rows match this site's other IRC-based calculators; the open-riser and guard rows are marked [VERIFY] because they vary more by local amendment.
Cantilever, structural sign-off & what this calculator can't tell you
A floating stair can fail for reasons a conventional stair never has to think about — none of them show up in a riser/tread/angle check.
Why cantilever length is the real budget driver
Every inch of unsupported cantilever multiplies the bending load the hidden steel has to resist. Past a rule-of-thumb threshold — commonly cited around 12–14" per side for a center stringer [VERIFY] — fabricators typically require a stamped engineering calculation before they'll build it.
Open riser gap needs a real code check
This calculator computes the gap and flags it, but the exact sphere size and rise-based exception vary by jurisdiction and adopted code year [VERIFY]. Treat the amber chip as "go check this," not "this failed."
Deflection and bounce, not just static load
A stair can pass every static load and dimension check and still feel springy underfoot if the support beam or wall engagement isn't stiff enough. This is a structural engineering question this calculator does not answer — it's the single most common source of buyer's remorse on floating stairs.
When you need a landing
IRC still caps a single flight at 147" (12'-3") of rise even when there are no risers to look at. This calculator's Landings Needed result and the compliance chip flag it the same way as any other stair on this site.
These are planning-stage estimates, not a stamped structural design. Every floating stair — regardless of support type or material — should get a structural engineer's review before fabrication; the cantilevered, open-riser nature of this stair type removes the built-in redundancy a closed, fully-supported stringer stair has.
Common mistakes when planning a floating stair
Most floating-stair problems trace back to a design decision made before anyone talked to a fabricator, not to bad execution.
Assuming "floating" means less structure
A floating stair almost always needs more steel than a boxed stair, not less — the support just moves somewhere hidden instead of disappearing. Budget for a heavier structural line item, not a lighter one.
Picking stair width for looks before checking cantilever
A dramatic 42"–48" wide tread on a standard center beam can push cantilever well past what most fabricators will build without an engineer. Decide width and support type together, not width first.
Ignoring the open-riser sphere rule until inspection
It's easy to assume any open gap is fine because the stair "looks" like others you've seen. The sphere rule and its small-stair exception are code, not style — check them before treads are ordered, not after.
Treating tread thickness as a finish decision
On a floating stair, tread thickness is structural, not aesthetic. Choosing a thin tread to match a "minimal" look without confirming it can actually carry the cantilevered load is how treads end up needing a hidden steel core added after fabrication starts.
Designing for static load only
A calculation that satisfies code minimum load capacity can still produce a stair that visibly deflects or feels bouncy in daily use. Ask your engineer about deflection limits specifically, not just pass/fail load capacity.
Skipping wall blocking on a wall-mounted design
Wall-mounted cantilever treads need solid blocking or an engineered ledger between studs at every single tread location, decided before drywall goes up. Finding this out during finish carpentry means opening the wall back up.
When to use a floating staircase calculator
Floating stair planning shows up earlier in the design process than a conventional stair, because the support decision shapes the framing and finish budget both.
Modern Home Renovation
Replacing a boxed stair with an open, cantilevered design in a great room or entry.
New Custom Construction
Working out rise, run and support type with an architect before framing is priced.
Loft & Mezzanine Access
Fitting an open-riser stair into a tight footprint without visually blocking the space below.
Fabricator & Engineer Quoting
Getting rough cantilever and tread numbers together before requesting a stamped structural quote.
High-End Spec & Flip Upgrades
Comparing support systems against budget before committing to a signature staircase feature.
Permit & Plan Check Prep
Checking open-riser gap and landing requirements before submitting for plan review.