How the sheet count is worked out
Wall area is the room's perimeter times the wall height, 2 × (length + width) × height, minus any openings you choose to leave out. Tick the ceiling to add length × width. The total is increased by the waste percentage and divided by the area of one sheet: 32 square feet for a 4 × 8, 40 for a 4 × 10 and 48 for a 4 × 12. The result is rounded up, because a partial sheet is still a sheet you have to buy.
Finishing materials
Joint compound, tape and screws scale with the board area. The calculator uses USG's published figures of about 9.4 gallons of ready-mixed compound and 370 feet of paper tape per 1,000 square feet of board, which assume the usual three coats: tape, fill and finish. Screws are estimated at one per square foot, a common rule of thumb for screws every 12 to 16 inches along each stud or joist. Corner bead is not included; measure every outside corner and buy it in 8 or 10 foot lengths.
Choosing the inputs
Most drywall pros do not subtract windows and doors, because the cut-outs rarely fit anywhere else. Leave openings at zero unless a wall is mostly glass or a large patio door. 10% waste is normal for a simple rectangular room; use 15% for rooms with many corners, soffits or angled ceilings. Hang the longest sheets you can carry, horizontally on walls, to cut down on butt joints.
What this does not cover
The calculator assumes one layer of standard board on a rectangular room. Moisture-resistant board for bathrooms, fire-rated Type X for garages, and double layers for soundproofing are counted the same way but priced differently, so run them separately. Corner bead, primer and texture are separate items.