How drywall sheet calculator works
Drywall estimating is an unwrapping problem: a room's walls unfold into one long rectangle whose length is the perimeter — 2 × (length + width) × height — while the ceiling is simply length × width. A 12 × 14 ft bedroom with 8 ft ceilings works out to 416 sq ft of wall and 168 sq ft of ceiling.
Openings come out next, using standard approximations: 21 sq ft per door (3 × 7 ft) and 15 sq ft per window (3 × 5 ft). One door and two windows leave 533 sq ft of board to hang. Then the waste multiplier — 10% by default, 15% if the room has many corners or you're new to hanging — covers the sheet you measure wrong, cuts around electrical boxes, and the corner you break carrying it.
Sheets are 4 ft wide and 8, 9, 10 or 12 ft long, so one sheet covers 32, 36, 40 or 48 sq ft. The calculator divides gross area by sheet coverage and rounds up to whole sheets — you can't buy 0.32 of a sheet, and leftovers beat a second store run. Screws are estimated at one per square foot, which matches the standard pattern: 12 inches on center in the field, 8 inches at sheet edges, landing near 32 screws per 4×8 sheet.