Drywall Calculator
Updated 2026-10-14
Enter the size of the room, the number of doors and windows, and the sheet size you plan to use. The calculator works out the area to cover and the number of sheets to buy, including a waste allowance.
How the drywall calculator works
The walls of a room add up to its perimeter (the distance all the way around) multiplied by the wall height. The calculator subtracts standard-sized doors and windows, adds the ceiling if you include it, then adds a waste allowance for offcuts. Finally it divides by the area of one sheet and rounds up.
Total area = wall area + ceiling (length × width)
Sheets = total area × (1 + waste %) ÷ sheet area, rounded up
A standard door is counted as 21 sq ft (3 × 7 ft, or 1.9 m²) and a standard window as 15 sq ft (3 × 5 ft, or 1.4 m²). Many installers don't subtract small windows at all, because the piece cut out is usually waste anyway. If you want an extra margin, set the number of windows to 0.
Worked example
A 12 × 10 ft room with 8 ft walls has a perimeter of 44 ft, so the walls are 352 sq ft. Take away one door (21 sq ft) and two windows (30 sq ft) and the walls come to 301 sq ft. Add the 120 sq ft ceiling for a total of 421 sq ft. With 10% waste that's 463 sq ft, and with 4 × 8 ft sheets covering 32 sq ft each, you need 14.5 sheets, so buy 15.
Which sheet size should I use?
| Sheet size | Area | Best for |
|---|---|---|
| 4 × 8 ft (1.2 × 2.4 m) | 32 sq ft (2.88 m²) | DIY, small rooms, easy to carry and fit in a car |
| 4 × 10 ft (1.2 × 2.7 m) | 40 sq ft (3.24 m²) | Walls slightly taller than 8 ft |
| 4 × 12 ft (1.2 × 3.0 m) | 48 sq ft (3.6 m²) | Long walls and ceilings: fewer joints to tape, but heavy and needs two people |
Fewer, longer sheets mean fewer seams, and seams are where most of the finishing time goes. Metric sheet sizes vary by country, so check the exact size at your supplier and choose the closest option.
Thickness and type
For walls, 1/2 inch (12.5 mm) board is the usual choice. Ceilings are often done with 1/2 inch sag-resistant board or 5/8 inch (15 mm) board, especially where joists are widely spaced. Bathrooms and other damp areas need moisture-resistant board, and some locations, such as walls between a garage and a house, need fire-rated board by building code. Check your local code for the specific requirements where you live.
Other materials you'll need
- Screws: drywall screws long enough to go about 5/8 in (15 mm) into the studs. Buy a box rather than a small pack.
- Joint tape and compound: paper or mesh tape for the seams, and joint compound for taping, filling screw holes and finishing coats.
- Corner bead: metal or paper-faced bead for every outside corner. Measure the height of each corner to know how many lengths to buy.
- Primer: new drywall needs a coat of drywall primer before painting. Our paint calculator works out how much.
Frequently asked questions
- How many sheets of drywall do I need for a 12 × 12 room?
- With 8 ft walls, one door and two windows, the walls are 333 sq ft and the ceiling 144 sq ft, so 477 sq ft in total. With 10% waste that's about 525 sq ft, or 17 sheets of 4 × 8 ft.
- How many sheets of drywall for a 10 × 10 room?
- With 8 ft walls, one door and two windows, the walls are 269 sq ft. Add the 100 sq ft ceiling for 369 sq ft. With 10% waste, that's 13 sheets of 4 × 8 ft.
- Should drywall be hung horizontally or vertically?
- On walls up to about 9 ft high, hanging sheets horizontally usually means fewer and easier-to-finish seams. The number of sheets you need is about the same either way.