How to use the drywall calculator
- Enter the room length, width and wall height.
- Choose whether to include the ceiling.
- Enter the number of doors and windows.
- Pick the sheet size you plan to buy and keep about 10% waste for cuts.
The calculator subtracts 21 sq ft for each door (3 × 7 ft) and 15 sq ft for each window (3 × 5 ft). If your openings are much larger, such as patio doors, count them as two. Count doors and windows room by room; on Decoratoradvice we have found they make a noticeable difference to the sheet total.
Drywall formula
Wall area = 2 × (length + width) × wall height
Ceiling area = length × width
Net area = wall area + ceiling area − openings
Sheets = net area × (1 + waste) ÷ sheet area
Sheet areas: 4 × 8 ft = 32 sq ft, 4 × 10 ft = 40 sq ft, 4 × 12 ft = 48 sq ft.
Worked example
A 12 × 12 ft bedroom with 8 ft walls, one door and one window, walls and ceiling:
Walls: 2 × (12 + 12) × 8 = 384 sq ft
Ceiling: 12 × 12 = 144 sq ft
Openings: 21 + 15 = 36 sq ft
Net area: 384 + 144 − 36 = 492 sq ft
With 10% waste: 541 sq ft ÷ 32 = 16.9, so 17 sheets of 4 × 8
Walls only, with 4 × 12 sheets and no waste allowance, the same room needs 348 sq ft, or 8 sheets.
Choosing a sheet size
Longer sheets mean fewer joints to tape, which saves finishing time. A 12 ft sheet can cover a 12 ft wall with no butt joint. The trade-off is weight and handling: long sheets are hard to carry up stairs and through doorways, and may need two people or a lift for ceilings.
Tips
- Plan sheet layout on a sketch for rooms with many openings. You can often cut around a window from one sheet.
- Hang the ceiling first, then the walls, so the wall sheets support the ceiling edges.
- Buy an extra sheet beyond the estimate for damaged corners. It costs less than a second trip.
- Measure to the nearest inch and round wall heights up, not down.
Common mistakes
- Subtracting every opening at full size. Offcuts around small windows are rarely reusable, which is why the estimate keeps a waste allowance.
- Forgetting closets and soffits. Add their wall area separately.
- Using floor area only. Walls usually make up most of the area: in the example, walls are 384 of 528 sq ft before openings.
Frequently asked questions
How many sheets of drywall do I need for a 12 × 12 room?
With 8 ft walls, a ceiling, one door, one window and 10% waste, 17 sheets of 4 × 8. Walls only, 12 sheets of 4 × 8.
How much does one sheet of drywall cover?
A 4 × 8 sheet covers 32 sq ft, a 4 × 10 covers 40 sq ft and a 4 × 12 covers 48 sq ft.
Should I subtract doors and windows?
Yes for an estimate, but keep a waste allowance because the pieces cut out are often unusable. This calculator does both.
How much waste should I allow?
About 10% for simple rooms and 15% for rooms with many corners, openings or angled ceilings.
What do I need after hanging drywall?
Taping, mud and sanding, then primer and paint. The paint calculator works out paint from the same room dimensions, and the baseboard calculator finishes the bottom edge.
How do I know where the studs are?
Studs are usually 16 or 24 in apart. The stud calculator shows how many a wall needs, which also tells you how many screw lines each sheet will cross.
Key takeaways
- Net area = walls + ceiling − openings.
- Divide by the sheet area after adding 10–15% waste.
- A 12 × 12 room with 8 ft walls and a ceiling takes 17 sheets of 4 × 8.
- Longer sheets mean fewer joints but are harder to handle.