Measuring the void before you calculate
This tool reuses the same three numbers your stair layout already produced. Get those right and the storage numbers follow automatically.
Pull your rise and run from the stair layout
Total Rise and Total Run are the same floor-to-floor height and floor length you'd enter on the Rise and Run Calculator — the void beneath the stair is the mirror image of the stair itself.
Measure the clear width
Measure between whatever will bound the storage space on each side — the outer stringers if it's staying open, or the planned framed walls if it's being built into a closet.
Set a clearance height that matches your plan
Decide how low a ceiling you're willing to shelve or crawl into. There's no code answer here — it's a judgment call between usable depth and comfort.
Check the fit and compliance chips
The Item Fits chip confirms your tallest planned bin clears the sloped ceiling at the depth you'd actually place it, and the fire-separation chip is a reminder to confirm local code before finishing the enclosure.
The four formulas behind every result
The void under a stair is a right triangular prism with a sloped ceiling. Every result below comes from slicing that shape at your chosen clearance height.
Full Triangular Void Volume
The complete wedge beneath the stair, before any framing or clearance limit is applied — the ceiling-to-floor volume if you could use every cubic inch.
Usable Depth
How far in from the tall end the sloped ceiling stays at or above your clearance height, measured along the floor.
Net Usable Volume
The trapezoid-shaped slice from the tall end back to the clearance line, times the width remaining after knee-wall and finish losses.
Interior Height, Front
The real clear height at the tallest point once a sloped ceiling is furred to follow the underside of the stringer — always less than the raw rise number.
Walking through a 108" rise, 130" run void
Same rise and run as the site's Rise and Run Calculator worked example, so the two pages agree on the same staircase.
Full void volume
Converting to feet: 9' rise, 10.83' run, 3' width. ½ × 10.83 × 9 × 3 = 146.3 ft³ — the entire wedge, before any deductions.
Usable depth at a 48" clearance
d = 10.83 × (1 − 48÷108) = 10.83 × 0.556 ≈ 6.02 ft (72.2") back from the tall end before the ceiling drops below 48".
Net usable volume
With a 4.5" knee wall on one side, net width is 2.625'. Trapezoid area = ½ × (9 + 4) × 6.02 ≈ 39.1 ft². × 2.625' width ≈ 102.7 ft³ — this is the number worth building shelving for.
Interior height at the front
108" − 11.25" stringer − 0.5" drywall = 96.3" (8.02') of real clear height at the doorway — over a foot less than the raw 108" rise, which is the number people mistakenly design around.
This is exactly why a raw rise number oversells the space — the stringer depth and ceiling finish come off the front height, and the clearance line cuts the back off entirely. A wider knee wall or a lower clearance line both push the net volume number up or down significantly; try both in the Advanced panels.
What different clearance heights are good for
These are commonly used space-planning categories, not code figures — there is no code minimum height for storage space. Pick the one that matches how you'll actually use it.
| Clearance Range | Typical Use | Access Style |
|---|---|---|
| Under 24" | Flat bins slid in from the open side; seasonal decorations | Crawl / reach-in only |
| 24"–47" | Stacked totes, luggage, vacuum and cleaning gear | Stoop and reach |
| 48"–65" | Coat closet, pantry shelving, small appliances | Bent-over standing |
| 66" and up | Walk-in closet, half-bath, wine storage, home office nook | Full standing |
What code actually regulates here
Building code has almost nothing to say about how much storage volume you get — it regulates fire separation for any usable enclosed space under a stair, which is the one figure worth confirming locally.
| Item | Typical Requirement | Notes |
|---|---|---|
| Fire-separation of enclosed usable space under stairs | [VERIFY: exact IRC section & gypsum thickness for your adopted code edition] | Commonly cited as ½" gypsum board on the enclosed side under IRC — section numbering and thickness vary by code edition and local amendment. |
| Minimum usable storage clearance | No code minimum | Purely a design choice — see the Clearance Categories table above. |
| Egress / handrail impact | Not applicable | Storage below the stair doesn't affect the stair's own tread, riser or handrail compliance above it. |
Reference only — always confirm the fire-separation requirement against your local jurisdiction's adopted code before enclosing the space.
Framing, fire-rating & access
A void with plenty of raw volume can still make a poor closet if these four things aren't planned for.
Fire-separation on the enclosed side
Enclosing the void into usable space typically triggers a fire-rated gypsum requirement on the stair side, protecting anyone using the stair above from a fire that starts in the storage. Confirm the exact thickness and section for your adopted code edition before drywalling.
Stepped shelving vs. sloped ceiling
Instead of one continuous sloped ceiling, many builders frame 2–3 stepped rectangular bays that follow the stringer in stages. It sacrifices a little volume near each step-down but makes shelving dramatically easier to build square.
Door and access choice
A hinged door needs its full swing arc clear on both sides, which eats directly into your Usable Floor Area. A sliding, bifold, or pocket door reclaims that area — enter 0 in Door Swing Area to model that option.
Structural notching limits
It's tempting to notch into the stringer itself to gain another inch of shelf depth. Don't — stringers are structural members, and cutting into the bottom (tension) edge can weaken the stair above. Build storage framing independent of the stringers.
These are planning-stage estimates, not a stamped structural or fire-code design. A permit-triggering renovation should get a local plan check before you cut drywall.
Common mistakes when planning under-stair storage
Most disappointing under-stair closets trace back to one of these five planning errors, not to bad carpentry.
Designing around the raw rise number
The stringer depth and ceiling finish always eat into the front interior height. Plan shelving and door height against Interior Height, Front — not the raw Total Rise.
Ignoring the back of the wedge entirely
The last few feet of a long run drop to near-zero height. Treating the whole floor footprint as shelvable overstates capacity — use Usable Depth, not Total Run, for shelving layout.
Framing a hinged door without checking swing area
A hinged door can consume several square feet of the shallow-height zone right at the entrance — often the most usable part of the space. Model it with the Door Swing Area field before committing.
Notching the stringer for extra depth
Cutting into the stringer's bottom edge to steal an inch of shelf space compromises the structural member carrying the stair above. Build the shelf frame off the floor and side walls instead.
Skipping the fire-separation check
An enclosed usable space under a stair commonly needs fire-rated gypsum board on the stair side under residential code. Confirm the requirement with your local building department before finishing the enclosure.
When to use an under-stair storage calculator
Under-stair space is one of the most commonly wasted footprints in a home — this shows up whenever someone is finally deciding what to do with it.
Basement Finishing
Turning an open stair void into a finished closet during a basement remodel.
Custom Closet Design
Sizing shelf bays and drawer units before ordering closet components.
Wine or Pantry Nook
Checking whether the void is deep and tall enough for a small wine rack or pantry shelving.
Pet Nook Planning
Sizing a crawl-in pet bed or litter box area that only needs a low clearance zone.
Utility & Vacuum Storage
Confirming a vacuum, mop bucket, or tool caddy fits at the depth it would actually sit.
New Construction Planning
Deciding at framing stage whether to box the void in as a closet or leave it open.