How many sheets of drywall a room needs
Drywall is sold in 4-foot-wide sheets that are 8, 10 or 12 feet long, covering 32, 40 or 48 sq ft each. To estimate, add up the area to be covered, add waste, and divide by the sheet size. The wall area is the room perimeter times the height: 2 x (12 + 12) x 8 = 384 sq ft for the default room. Include the ceiling if it is being hung too, which adds length x width, or 144 sq ft here. Openings can be deducted at 20 sq ft per door and 15 sq ft per window, but many contractors skip that deduction because the pieces cut out around openings are rarely reusable; the checkbox lets you choose. With deductions, 384 + 144 - 50 = 478 sq ft.
Waste of 10% covers cuts, breakage and the odd sheet you get wrong. That brings the example to 526 sq ft, and 526 / 32 = 16.4, so buy 17 sheets of 4 x 8. Choosing 4 x 12 sheets gives 526 / 48 = 11 sheets and far fewer joints to tape on a 12-foot wall, which is why pros prefer long sheets whenever they can get them through the door. Sheets are heavy (a 1/2-inch 4 x 8 weighs about 50 lb; 4 x 12 about 77 lb), so plan on two people or a lift for ceilings.
Joint compound, tape and screws
The finishing materials are estimated with rules of thumb that the big-box store calculators also use. Joint compound: a 4.5-gallon bucket of all-purpose or lightweight mud (about 62 lb, or 61.7 lb in the classic size) finishes roughly 450 sq ft of drywall through three coats, so 526 sq ft needs 1.2 buckets, rounded up to 2; a smaller job might get by with a 3.5-gallon box. If you also skim-coat or use a heavy texture, double it. Joint tape: about 37 linear feet per 100 sq ft, so this room uses close to 195 feet, which one 250-foot roll of paper tape covers; mesh tape comes in 300-foot rolls and needs setting-type compound for the first coat.
Screws: with studs and joists on 16-inch centers and screws every 12 inches in the field and 8 inches on the edges, a 4 x 8 sheet takes about 32 screws, or one per square foot. Seventeen sheets is about 544 screws; a 1-lb box of 1-1/4-inch coarse-thread drywall screws holds about 250-300, so buy two, or one 5-lb box if there is more drywall in your future. Add corner bead for every outside corner (one 8-foot stick each) and a few tubes of construction adhesive if you want fewer screws and quieter walls.
Choosing thickness and other details
Half-inch drywall is standard for walls and for ceilings framed 16 inches on center. Use 5/8-inch on ceilings framed 24 inches on center (it resists sagging) and wherever a fire rating is required, such as a garage wall shared with the house. Moisture-resistant greenboard or, better, cement board or a waterproof membrane belongs in tub and shower surrounds, and mold-resistant board is a good idea in bathrooms and laundry rooms. Whatever you choose, order a sheet or two beyond the estimate; a return trip costs more in time than a spare sheet does in dollars, and a leftover sheet is handy for the first patch job.