Drywall is the project where running short sends you to the hardware store mid-job, and over-ordering means you're sliding 4' × 12' sheets back into the truck through the bedroom window. The math is easy. What separates a clean wall from a DIY-looking wall is what you put on top of it — which mud, which tape, and the rule about inside corners that pros never break.

Sheet count — the formula

Sheets = ceil( (wall_area − openings) / sheet_area × 1.10 )

  wall_area    = perimeter × ceiling_height + ceiling_area
  openings     = door area + window area + cased openings
  sheet_area   = 32 sf (4×8), 40 sf (4×10), 48 sf (4×12)
  × 1.10       = 10% waste factor (cuts, broken corners,
                 the one you'll over-rotate against the
                 stairwell on day one)

On rooms with a lot of small windows or short walls, push the waste factor to 15%. On a clean rectangular basement with two openings, 10% is enough.

Pick the sheet size

• 4' × 8' — the only choice if you're carrying it up a stairwell or working alone. ~54 lb each.
• 4' × 10' — the sweet spot for most rooms with 9-ft ceilings. One sheet does a 10-ft wall floor-to-ceiling with no horizontal seam.
• 4' × 12' — fewer butt joints (the joints you can't hide). Required gear: a panel lift, or two strong people. Do not attempt to hang 4×12 5/8" ceiling board solo.
• 1/2" standard — walls and ceilings up to 16" oc framing.
• 5/8" Type X — required on garage ceilings under habitable space (IRC R302.6) and on 24" oc ceiling framing (sags otherwise). Also required for fire-rated walls.
• USG UltraLight — 25–30% lighter for the same span rating. Worth it once you're past two rooms.

Fasteners — screws, not nails

Walls (16" oc framing):
  - Field screws every 16" along each stud
  - Edge screws every 8"
  → Roughly 32 screws per 4×8 sheet, 40 per 4×10, 48 per 4×12

Ceilings (16" oc):
  - Field every 12" along each joist
  - Edge every 8"
  → Roughly 40 screws per 4×8, 50 per 4×10, 60 per 4×12

Length:
  - 1-1/4" coarse for 1/2" board on wood studs
  - 1-5/8" coarse for 5/8" board on wood studs
  - FINE thread for metal studs (coarse spins on steel)

Mud quantity — the rule of thumb

• One 4.5-gallon bucket covers about 400–500 sf of finished wall (tape + fill + skim, three coats).
• One roll of paper tape (250') does about 500 sf of wall.
• One bag of USG Easy Sand 90 (5 lb) is enough to pre-fill the deep gaps on a typical room. Buy two; you'll use them.
• The math is forgiving — buy 10–15% extra mud. Unopened buckets keep for a year; the second trip costs you a half day.

The paper-vs-mesh rule (don't break this one)

The single most-cited DIY mistake in drywall finishing is using fiberglass mesh tape on inside corners. It cracks within one heating season. Inside corners get compression movement; only paper tape, with its center fold crease, can absorb it without telegraphing a crack.

• Paper tape — every inside corner, every flat seam, every tapered seam. Bedded in regular all-purpose mud.
• Mesh tape — small flat patches only, and only when paired with hot mud (Easy Sand). Never inside a corner. Never on a butt joint that will see daily-life stress (over a doorway, behind a couch, near a heat register).
• FibaFuse (a hybrid mesh-paper tape from Saint-Gobain ADFORS) — a defensible alternative for stress-prone flats. Stronger than paper, doesn't crack like mesh. Pros are split on it; the consensus floor is "at least as good as paper."

Worked example: 12 × 14 bedroom, 8-ft ceiling, one door + one window

Walls:
  Perimeter:    2 × (12 + 14) = 52 ft
  Wall area:    52 × 8 = 416 sf

Ceiling:
  12 × 14 = 168 sf

Openings:
  Door:         3 × 6.67 = 20 sf
  Window:       3 × 4    = 12 sf
  Total:        32 sf

Net area:       416 + 168 − 32 = 552 sf

Sheets (1/2", 4×8 = 32 sf):
  552 / 32 × 1.10 = 19.0 sheets → buy 19

Sheets (1/2", 4×10 = 40 sf):
  552 / 40 × 1.10 = 15.2 sheets → buy 16

Mud:           552 / 450 = 1.2 buckets → buy 2 × 4.5 gal
Tape:          552 / 500 = 1.1 rolls   → buy 2 × 250'
Hot mud:       2 bags Easy Sand 90 (pre-fill)
Screws:        ~700 screws (1 lb box covers ~250) → buy 3 lb

Hang the ceiling first. Always.

Wall sheets butt up tight to the ceiling and help carry the load. Hanging walls first means trying to hold a 12-ft sheet of 5/8" Type X up against the joists with one hand while you screw it off with the other, which is how people end up with 1/4" gaps along the top of every wall and a sheet on the floor that's now in three pieces. Rent a panel lift ($30/day at any rental yard) for solo ceiling work — it pays for itself in not destroying sheets, never mind your back.

Three coats, in order

• Coat 1 — tape coat (6" knife): Skim mud into the joint, embed paper tape, squeegee the excess back out. Too little = bubbles. Too much = ridges you'll sand for an hour.
• Coat 2 — fill coat (10" knife): Wider, smoother. Feather edges 6+ inches past the joint. Skim screw heads same time. Wait 24 hours between coats.
• Coat 3 — skim coat (14" knife): Thin, wide, flat. Wider knife = flatter look. Feather to nothing.
• Sand under raking light. Pole sander, 120 then 220. Hold a flashlight or headlamp flat against the wall — every ridge shows. Skip the flashlight check = skip having a flat wall.

Common mistakes

• Mesh tape inside corners. Cracks within one season. Use paper.
• Butt joints over door or window heads. Highest-stress spot on the wall — the joint cracks within a year, guaranteed. Plan the layout to avoid this.
• Over-tightening screws (breaking the paper face). Screw has zero holding power. Leave it; drive a new screw 2" away.
• Mudding over wet mud. Cracks through every coat above it. 24 hours between coats, every coat.
• 1/2" drywall on a 24" oc ceiling. Sags. Use 5/8" or sag-resistant ceiling board.
• Skipping the N95. Drywall dust is silica. Lungs really do hate it.

Run the takeoff on your phone

The BuilderCalc app's drywall calculator does this exact math — wall area minus openings, sheets in your chosen size with waste factor, screws, mud, and tape — offline, free on the App Store and Google Play. The Install drywall project guide walks the hang-tape-mud-sand sequence end to end, with the brand-specific picks (USG green vs blue lid, Easy Sand 90, paper-faced corner bead) wired in.

Related

• BuilderCalc — DIY project guide + 14 calculators
• How much paint do I need? The math + brand picks

Note: Numbers above are starting points. Local codes govern fire-rating, garage ceilings, and any wet-area substrate (greenboard / cement board / Schluter Kerdi for wet walls). Verify against your AHJ and the manufacturer's installation guide.