Openings are not deducted; the offcuts usually get used around them.
Sheet sizes
A 4 × 8 ft sheet covers 32 sq ft and a 4 × 12 ft sheet covers 48 sq ft. Larger sheets mean fewer seams to tape but are heavier to handle. Half-inch board is standard for walls; 5/8-inch Type X is fire-rated and is often required on garage walls shared with a house.
Worked example
A 12 × 14 ft room with 8 ft walls has 416 sq ft of wall. Subtract about 20 sq ft per door and 15 sq ft per window, add 10% waste, and divide by 32 for the number of 4 × 8 sheets.
Frequently asked questions
How many sheets for a 12×12 room?
About 14 sheets of 4×8 for 8 ft walls, plus 5 for the ceiling.
Should I subtract doors and windows?
Yes, but many installers keep a small extra for cut-outs; the waste allowance handles that.
Do ceilings need different board?
Ceilings often use 5/8-inch or sag-resistant 1/2-inch board.