Estimate drywall sheets and joint compound from your wall and ceiling area, minus openings, with a waste factor.
sheets = (wall + ceiling − openings) ÷ 32 × (1 + waste)
A 4×8 sheet covers 32 ft²; a 4×12 covers 48 ft² and gives cleaner ceiling joints.
Use 10% waste for simple rooms, 15% with many openings. Plan 25–30 lb of joint compound per 100 ft² for a Level 4 finish.
Walls: perimeter is 2 x (14 + 16) = 60 ft, times 9 ft high = 540 square feet of wall. Ceiling: 14 x 16 = 224 square feet. Subtract a door (about 21 sq ft) and two windows (about 30 sq ft) for 51 square feet of openings. Net surface: 540 + 224 − 51 = 713 square feet.
Using 4 x 8 sheets (32 sq ft each): 713 ÷ 32 = 22.3 sheets, and at 10% waste that's about 25 sheets. You'll also need joint compound — at roughly 27 lb per 100 square feet for a Level 4 finish, that's about 190 lb, or four to five buckets.
Cutouts at doors, windows, and outlets generally can't be reused, so the opening area does not reduce your sheet count by its full amount — the waste factor covers this.
About 17 sheets of 4×8 including 10% waste, for roughly 478 ft² of wall and ceiling.