Short answer: a 12′ × 14′ room with a 9′ ceiling needs ~600 sq ft of drywallonce you net the door and one window. That's 19 sheets of 4′×8′ (32 sq ft each), 17 sheets of 4′×9′ (36 sq ft each), or 13 sheets of 4′×12′ (48 sq ft each). Order one extra sheet past the calc, plus 1 box of screws (5 lb), 1 box of mud (3.5 gal), and 250 ft of paper tape. Below is the math, the sheet-size decision, and the spec for a 9′ ceiling specifically — which is where most online calculators get the answer wrong.

The formula

Wall area      = perimeter × ceiling height
Ceiling area   = length × width
Total area     = wall area + ceiling area
Net area       = Total area − openings
Sheets         = Net area / sq ft per sheet, round UP, +1 spare

Worked example: 12′ × 14′ × 9′

Walls:
  Perimeter = 2(12 + 14) = 52 ft
  Wall area = 52 × 9     = 468 sq ft

Ceiling:
  Ceiling area = 12 × 14 = 168 sq ft

Gross area:
  468 + 168              = 636 sq ft

Subtract openings:
  Standard door (3' × 7') = 21 sq ft
  Standard window (3'×5') = 15 sq ft
  Total openings           = 36 sq ft

Net area:
  636 − 36               = 600 sq ft

Sheet count by size:
  4'×8'  (32 sq ft):    600 / 32 = 18.75 → 19 sheets + 1 spare = 20
  4'×9'  (36 sq ft):    600 / 36 = 16.67 → 17 sheets + 1 spare = 18
  4'×10' (40 sq ft):    600 / 40 = 15.00 → 15 sheets + 1 spare = 16
  4'×12' (48 sq ft):    600 / 48 = 12.50 → 13 sheets + 1 spare = 14

Why 4′×9′ or 4′×12′ for a 9′ ceiling

Drywall hung horizontally (long edge perpendicular to studs) is the standard install in most US framing — it puts the seam in the middle of the wall, perpendicular to gravity, where it's easiest to tape and float. With an 8′ wall, two 4′×8′ sheets stack perfectly: bottom sheet covers 0–4′, top sheet covers 4–8′.

At a 9′ wall, two 4′×8′ sheets only cover the first 8′. You need a 12′×1′ rip on top of every wall — meaning two extra horizontal seams per wall, multiplied by four walls = eight extra seams, each ~12′ long. That's 96 linear feet of extra seam to tape and mud, just because you used the wrong sheet size.

With 4′×9′ sheets hung vertically, each sheet covers floor to ceiling in one piece. Zero horizontal seams. The 12′ sheet does the same thing more efficiently — a 12′ horizontal sheet on a 14′ wall covers most of it in one piece with a single 2′ vertical butt at the end of the wall.

Sheet thickness for a 12 × 14 room

• 1/4" sheet — only for skim-coating curved walls or layering over existing drywall.
• 3/8" sheet — only for double-layer or curved-wall applications.
• 1/2" standard — the default for residential walls and ceilings — this article's spec.
• 1/2" lightweight (USG Sheetrock UltraLight) — same coverage, ~25% less weight. Worth the few extra cents.
• 5/8" Type X — required by IRC R302.6 for garage ceilings under living space, attached-garage common walls, and party walls. Fire-rated.
• 1/2" or 5/8" Type C — high-rise or commercial fire-rated assemblies. Probably not what a 12×14 residential room needs.

Screws, mud, and tape (worked)

Screws (1¼" coarse for 1/2" drywall on wood):
  Spacing: 12" o.c. on edges, 16" o.c. in field (Code minimum)
  Practical install: ~1 screw per square foot
  600 sq ft × 1 = ~600 screws
  At ~250 screws / lb (1¼" coarse): 600 / 250 = 2.4 lb
  Order: 5 lb box (you'll use it on the next project too)

Joint compound (mud):
  Coverage: ~0.053 lb per sq ft of board (3 coats including taping)
  600 × 0.053 = 32 lb of mud
  3.5 gal bucket of pre-mixed all-purpose: ~50 lb usable
  Order: 1 bucket all-purpose (taping/topping)
         + ¼ bag setting-type 90 (joint embedding) — optional

Paper tape (250 ft per roll, ~$5):
  Linear footage of seams (typical):
    Horizontal seams (4'×9' vertical install): ~0
    Vertical seams (every 4'): perimeter / 4
    52 / 4 = 13 vertical seams × 9' = 117 LF
    Plus inside corners: 4 corners × 9' = 36 LF
    Plus ceiling/wall corners: 52 LF
    Total: ~205 LF
  One 250 ft roll = enough.

For 4'×8' horizontal install (NOT recommended at 9' ceiling):
  Add: 52 LF horizontal mid-seam + 52 LF horizontal top-seam = 104 LF more
  Plus rip-piece edges: 52 LF
  Total: ~360 LF — order 2 rolls.

Fasteners by sheet size

• 4′ × 8′ horizontal — ~36 screws per sheet.
• 4′ × 9′ vertical — ~38 screws per sheet.
• 4′ × 12′ horizontal — ~52 screws per sheet.
• Code minimum: 12" o.c. on edges (perimeter and butt), 16" o.c. in the field on walls. Ceilings get 12" o.c. everywhere.

The mud schedule (3 coats)

Coat 1 — TAPE coat (setting-type 90 or all-purpose):
  Embed paper tape in mud, knife smooth, leave wet.
  Working time: ~90 min for setting-type, indefinite for premix.

Coat 2 — FILL coat (all-purpose):
  Wider (8" knife) feathered out 6–8" past tape.
  Sand minimal between coats — knock down high spots only.

Coat 3 — SKIM coat (topping or all-purpose, thinned):
  Wider still (10–12" knife), feathered 12–14" past edges.
  This is the coat you sand most.

Total per seam: 3 coats, 24 hr dry between (less for setting-type).
Total schedule: tape Day 1, fill Day 2, skim Day 3, sand Day 4, prime Day 5.

9′ ceiling install: the practical sequence

• Hang ceiling first, perpendicular to joists. (4'×12' sheets if accessible — most efficient.)
• Hang walls top-down: top sheet first, butt to ceiling, push tight against tape on the ceiling perimeter.
• Bottom sheet up against top, leaving ~3/8" gap at the floor (covered by baseboard later).
• If using 4'×9' vertical: each sheet floor-to-ceiling in one piece. No mid-wall horizontal seam.
• Inside corners: factory edges meet, no tapered overlap. Embed corner with paper tape (or paper-faced metal corner if planning a clean look).
• Outside corners: paper-faced metal bead, embed with all-purpose, two skim coats.

The cost ballpark (12 × 14 × 9′)

4'×9' lightweight (UltraLight) install:
  17 sheets @ $13–16 ea       = $220–270
  5 lb screw box               = $12
  3.5 gal all-purpose mud      = $14
  250 ft paper tape            = $5
  Corner bead (4 outside)      = $20
  Total materials              ≈ $270–320

Comparable 4'×8' install (more seams):
  19 sheets @ $11–14 ea        = $210–270
  Same accessories              + extra tape ($5)
  Total                         ≈ $270–320

The cost is roughly the same. The labor delta is the win:
4'×9' saves 3–4 hours of taping and one extra mud day.

Common mistakes

• Buying 4'×8' sheets for a 9' wall and ripping to fit. Adds ~30 LF of butt joint per wall.
• Skipping the 1/4" gap at the floor. Drywall wicks moisture from the slab, then bubbles.
• Using mesh tape on butt joints (cracks within a year). Paper tape only.
• Sanding too aggressively between coats and exposing tape edges. Light sanding only between fill coats.
• Not priming before paint. Drywall paper and joint compound have different absorption — without primer, you can see every seam under one coat of finish.
• Forgetting Type X on a garage-attached common wall (IRC R302.6). Inspector will fail it.

Run it on your phone

The BuilderCalc app computes drywall sheet count, screws, mud, and tape for any room dimensions and ceiling height — with sheet-size optimization built in. 100% offline. Free on the App Store and Google Play.

Related

• Drywall sheets and mud — the full reference
• Drywall anchors and the 25-pound rule
• How many tiles for a 12×14 ft bathroom?
• BuilderCalc — construction calculator on iOS + Android

Note: Always verify door, window, and opening sizes against your actual rough framing — the "3′ × 7′ door" assumed above is nominal. Specialty rooms (wet baths, garage-attached common walls) may require greenboard or Type X drywall per IRC.