How the drywall calculator works
- Wall area = 2 × (length + width) × wall height.
- Subtract openings: 20 sq ft per door and 15 sq ft per window.
- Add the ceiling (length × width), if included.
- Add waste, then divide by the sheet area and round up to whole sheets.
Assumptions
- The room is rectangular with walls of equal height.
- Doors are counted as 20 sq ft and windows as 15 sq ft. Many pros skip subtracting small windows, because cut-outs rarely produce usable pieces.
- The screw count uses the common guideline of about one screw per square foot of board (about 32 per 4 × 8 sheet).
- Corner bead, joint tape and compound are not included.
Worked example
A 12 ft × 12 ft bedroom with 8 ft walls, one door, one window, the ceiling included, using 4 × 8 sheets and 10% waste.
- Walls: 2 × (12 + 12) × 8 = 384 sq ft, minus 20 + 15 for openings = 349 sq ft
- Ceiling: 12 × 12 = 144 sq ft, so the total is 493 sq ft
- With 10% waste: 542.3 sq ft ÷ 32 = 16.9, which rounds up to 17 sheets
Tips before you buy
- Longer sheets (4 × 12) mean fewer seams to tape, but they're heavy and harder to carry through doorways.
- Use 5/8 in board on ceilings when the joists are 24 in apart, and where fire-rated walls are required (such as garages).
- Use moisture-resistant board in bathrooms and laundry rooms.
Frequently asked questions
How many sheets of drywall for a 12×12 room?
With 8 ft walls and no openings, the walls take 384 sq ft, or 12 sheets of 4 × 8. With the ceiling added and 10% waste, about 19 sheets.
Should I hang drywall horizontally or vertically?
On walls, horizontal is common because it reduces seam length and bridges framing irregularities. Commercial work and tall walls sometimes go vertical.
How much waste should I allow for drywall?
10% is typical for simple rooms. Use 15% for rooms with many windows, angled ceilings or lots of small sections.