How drywall is estimated
Wall area = 2 × (length + width) × height − doors − windows
Ceiling area = length × width
Sheets = (wall + ceiling area) × (1 + waste %) ÷ sheet area, rounded up
Drywall (also called gypsum board, plasterboard or sheetrock) comes in sheets 4 ft wide, most commonly 8, 10 or 12 ft long. A 4 × 8 sheet covers 32 sq ft, a 4 × 12 sheet 48 sq ft. Longer sheets mean fewer joints to tape, but they are heavier and harder to carry into a house.
Worked example: 14 × 12 ft room, walls and ceiling
- Walls: 2 × (14 + 12) × 8 = 416 sq ft, minus one door (20) and two windows (30) = 366 sq ft.
- Ceiling: 14 × 12 = 168 sq ft. Total: 534 sq ft.
- With 10% waste: 534 × 1.10 = 587.4 sq ft.
- 4 × 8 sheets: 587.4 ÷ 32 = 18.4, so 19 sheets. With 4 × 12 sheets: 587.4 ÷ 48 = 12.2, so 13 sheets.
Should you subtract doors and windows?
Many professionals do not subtract small windows, because the cut-out piece is usually wasted anyway, and they treat it as part of the waste allowance. Subtracting large openings such as patio doors and wide windows is reasonable. If you are unsure, enter fewer windows: an extra sheet is cheaper than a second trip to the store.
Which thickness?
| Thickness | Typical use |
|---|---|
| 1/4 in | Covering existing walls, curved surfaces |
| 3/8 in | Repairs and double layers |
| 1/2 in | Standard for interior walls in houses |
| 5/8 in (often Type X) | Ceilings with wider framing, fire-rated walls (e.g. between garage and house where codes require it) |
Use moisture-resistant board in bathrooms and a cement or foam backer board behind tile in showers. Check your local building code for fire-rated assemblies.
Fasteners, tape and mud
Gypsum manufacturers' handbooks give screw spacing (commonly about 12 in on ceilings and 16 in on walls, depending on the assembly), and joint compound and tape quantities per 1,000 sq ft of board. Because these vary by product and assembly, use the figures in the manufacturer's guide for your board, and buy finishing materials in stages: it is easy to pick up another box of joint compound, harder to return opened ones.
Tips for fewer joints
- Hang the ceiling first, then the walls, top row before the bottom row.
- On walls up to 8 ft 1 in high, two 4 ft sheets laid horizontally cover the height with one horizontal seam.
- Choose sheet lengths that span a whole wall where possible, so there are no butt joints in the middle.
Step by step: estimating drywall
- Measure the length of each wall and the ceiling height.
- Note large openings such as doors, archways and big windows.
- Decide on the sheet length: one that matches your wall lengths or ceiling height reduces joints.
- Add the ceiling if it is being boarded, and remember closets.
- Add 10% for waste and round up to whole sheets.
Estimating the cost
Once you know the number of sheets, multiply by the price per sheet at your supplier. For example, if a 4 × 8 sheet costs 15.00, the 19 sheets for the room above cost 285.00 before fasteners, tape, joint compound and corner bead. Longer sheets cost more each but you need fewer of them, so compare the price per square foot.
Common mistakes
- Forgetting the ceiling or closets.
- Rounding sheets down. Always round up and keep a spare sheet for damage.
- Using the floor area for the walls. A 14 × 12 room has 168 sq ft of floor but 416 sq ft of wall at 8 ft high.
Frequently asked questions
How many sheets of drywall do I need for a 12 × 12 room?
With 8-ft walls the walls are 384 sq ft and the ceiling 144 sq ft, 528 sq ft in total. With 10% waste that is 580.8 sq ft, so 19 sheets of 4 × 8 or 13 sheets of 4 × 12 (before subtracting openings).
How many square feet is a sheet of drywall?
A 4 × 8 ft sheet is 32 sq ft, a 4 × 10 ft sheet 40 sq ft and a 4 × 12 ft sheet 48 sq ft.
Should I subtract windows when estimating drywall?
Subtract large openings such as patio doors. Many installers ignore small windows because the cut-out is waste anyway.
How much extra drywall should I buy?
About 10% for most rooms, 15% for rooms with many corners, angles or openings.
How do I convert drywall sheets to square metres?
A 4 × 8 ft sheet is 32 sq ft, which is 2.97 m². Metric sheets of 1,200 × 2,400 mm cover 2.88 m², so divide your wall and ceiling area in square metres by 2.88.
Sources
- USG, The Gypsum Construction Handbook (board sizes, thicknesses, fastener spacing and finishing).
- Gypsum Association, GA-216 Application and Finishing of Gypsum Panel Products.
Last reviewed 20 October 2026. Results are estimates; see our disclaimer.