Drywall Calculator
Sheet count for a room, walls and ceiling, with the waste allowance under your control.
The estimate
Wall area is the room perimeter times height, plus length times width if you're doing the ceiling. Divide by the sheet area (a 4x8 covers 32 sq ft) and add 10% for cuts and breakage. Openings are usually NOT subtracted unless they're large (a patio door, a pass-through): the cutouts mostly become unusable scrap anyway.
Which sheet size
4x12 sheets mean fewer joints to tape and finish, which is where the labor is, but they're heavy and awkward in small rooms and stairways. Ceilings usually get 1/2 in or 5/8 in sheets hung perpendicular to the joists; 5/8 in Type X is the standard where fire rating is required (garages, between units).