Example: A 10' x 10' room with 8' ceiling height. For this size room, each wall would be 80 square feet (10' wide x 8' tall = 80). You have four walls in the room, so 80 square feet x 4 = 320 square feet of drywall will be needed. Drywall typically comes in either 4' x 8' (32 square feet each) or 4' x 12' (48 square feet each) sheets. If your room requires 320 square feet of drywall, you'll just divide that by your sheet size to determine how many sheets will be needed.