How drywall sheets are calculated
Walls = 2 × (length + width) × height
Surface = walls + ceiling − doors − windows
Sheets = surface × (1 + waste %) ÷ sheet area, rounded upA door counts as about 21 sq ft and a window as about 15 sq ft.
Worked example: a 15 × 12 ft room with 8 ft walls
- Walls: 2 × (15 + 12) × 8 = 432 sq ft.
- Ceiling: 15 × 12 = 180 sq ft, for a total of 612 sq ft.
- Subtract one door and two windows: 612 − 51 = 561 sq ft.
- With 10% waste: 561 × 1.10 = 617.1 sq ft.
- In 4 × 8 ft sheets of 32 sq ft: 617.1 ÷ 32 = 19.3, so buy 20 sheets.
Which sheet size should you buy?
| Sheet | Area | Good for |
|---|---|---|
| 4 × 8 ft | 32 sq ft | Small jobs and rooms where one person is handling the sheets |
| 4 × 10 ft | 40 sq ft | Walls with 10 ft ceilings when hung vertically |
| 4 × 12 ft | 48 sq ft | Long walls and ceilings, where fewer seams mean less taping |
Longer sheets reduce the number of seams, but they are heavy and awkward. Before you order, check that they will fit up the stairs and through the doors, and consider renting a drywall lift for ceilings.
Should you subtract the openings?
Installers often hang sheets straight over small windows and doors, and then cut the opening out. Because the offcut is often too awkward to reuse, a window may save you less than its area suggests. If your room has many small openings, try entering fewer of them or raising the waste allowance.
What else goes on the list
- Drywall screws, joint compound, tape and corner bead are extra. Their packaging lists the coverage.
- Moisture-resistant or fire-rated board may be required in certain rooms, so check local code.
- Ceilings may call for thicker or sag-resistant board, depending on joist spacing.
Once the walls are finished, the paint calculator uses the same room measurements.