Sheet count is wall area divided by sheet size plus waste. The materials that come after — screws, compound and tape — are where published figures disagree wildly, and one widely copied number is off by roughly five times.
Joint compound: check the number you are given
USG, the manufacturer, publishes about 0.9 gallons of compound per 100 square feet of board for a standard three-coat finish. Working finishers report closer to one five-gallon bucket per ten sheets, which is about 1.5 gallons per 100 square feet.
Several calculator sites publish 5.3 gallons per 100 square feet. That is five times the manufacturer's figure and would have you buying a bucket for every 100 square feet of wall. We use 1.2 gallons as a middle figure, and you should expect to have some left over rather than run out.
Bigger sheets, fewer joints
Twelve-foot sheets on a twelve-foot wall eliminate the butt joint entirely, and butt joints are the hardest thing in drywall finishing to hide.
The trade-off is weight and handling. A 4 by 12 sheet of half-inch board is about 80 pounds and needs two people. If you are working alone, 4 by 8 is the practical limit.
Which thickness where
Half-inch is standard for walls. Five-eighths goes on ceilings — it sags less between joists — and is required on garage walls and ceilings attached to living space in most codes.
Moisture-resistant board belongs in bathrooms, but not behind tile in a shower. That needs cement board or a proper waterproof membrane system.
Screws, not nails, and not too deep
About one screw per square foot, roughly 32 per sheet, spaced 12 inches in the field and 8 along edges.
Set them just below the paper surface without breaking it. A screw that tears the paper holds nothing and has to be backed out and replaced an inch away.
Hanging ceilings first, and horizontally
Ceilings go up before walls, so the top edge of the wall sheets supports the ceiling edge rather than the other way round.
On walls, sheets are normally hung horizontally in residential work. It puts the long tapered seam at a comfortable height to finish, and it produces fewer linear feet of joint than vertical hanging does.
Butt joints — where two untapered ends meet — are the hard ones to hide, so they should be staggered between rows and kept away from the middle of a wall where light rakes across it.
Corner bead and the outside corners
Every outside corner needs bead, and it is a separate linear-foot item from the sheets. Metal bead is nailed or screwed, paper-faced bead is bedded in compound, and vinyl bead is stapled or glued.
Paper-faced is the most forgiving for a first-timer because it bonds to the compound and does not show a crack line where the bead meets the board.
Count the corners before ordering. A single room with a window return and a doorway can easily take fifty feet of bead, which is not obvious from a sheet count.