Wall area formula for rectangular rooms
Drywall quantity starts with wall square footage. For a standard rectangular room, wall area equals perimeter multiplied by ceiling height: Wall sq ft = 2 × (length + width) × ceiling height.
Example: a 12 ft × 14 ft bedroom with 8 ft ceilings has perimeter 52 ft. Wall area = 52 × 8 = 416 sq ft before openings. A 10 ft × 10 ft closet with 8 ft ceilings has 320 sq ft of wall — about nine sheets of 4×8 drywall with waste.
Ceiling drywall is calculated separately: ceiling sq ft = length × width. A 12×14 room adds 168 sq ft on top of wall area when you hang both surfaces.
Deducting doors, windows, and openings
Subtract rough openings so you do not over-buy. Rule of thumb for planning: standard interior door ≈ 20 sq ft, standard window ≈ 15 sq ft, double door or slider ≈ 40 sq ft.
For a precise takeoff, measure each opening (width × height) and subtract from total wall area. Large pass-throughs, fireplace bump-outs, and stairwells need individual measurements — our calculator accepts custom deduction per room.
- Single interior door (32×80): ~18–21 sq ft
- Window 3×4 ft: ~12 sq ft
- Garage service door: ~40 sq ft
- Skip tiny openings under 2 sq ft — waste covers them
Converting square feet to sheet count
Divide net area (plus waste) by sheet size. A 4×8 sheet covers 32 sq ft; a 4×12 covers 48 sq ft. Always round up to whole sheets — lumber yards do not sell half sheets.
416 sq ft of wall + 10% waste = 458 sq ft ÷ 32 = 14.3 → buy 15 sheets of 4×8. Switching to 4×12 sheets on long walls can drop the sheet count and reduce horizontal seams.
Multi-room projects and ordering
Add each room separately, then total sheets for one lumber-yard trip. Match sheet thickness and type (standard vs moisture-resistant) across the job so texture and paint absorb the same.
Use our drywall calculator for up to five rooms, ceiling toggle, custom sheet dimensions, and a copyable shopping list with screw and compound estimates.