How to use the drywall calculator
- Enter the room’s length, width and wall height.
- Count the doors and windows; a typical area is subtracted for each.
- Choose your sheet or board size and tick include the ceiling if you are boarding it too.
- Keep about 10% extra for cuts, offcuts and the odd damaged corner.
The drywall formula
Wall area = 2 × (length + width) × height − doors − windows Total = wall area + ceiling (length × width, if included) Sheets = total × (1 + waste % ÷ 100) ÷ sheet area (rounded up)
Doors are counted as about 20 ft² (1.6 m²) and windows about 15 ft² (1.5 m²).
Worked example (US)
A 12 × 12 ft room with 8 ft walls has 384 ft² of wall. With 10% extra (422 ft²) and 4 × 8 ft sheets (32 ft²), you need 14 sheets for the walls. Adding the 144 ft² ceiling takes it to 19 sheets.
Worked example (UK)
A 4 × 4 m room with 2.4 m walls and one door has 38.4 − 1.6 = 36.8 m² of wall. With 10% extra (40.5 m²) and 2400 × 1200 mm boards (2.88 m²), you need 15 boards.
Common sheet sizes
| Region | Size | Area | Typical use |
|---|---|---|---|
| US | 4 × 8 ft | 32 ft² | Most DIY jobs; easy to transport |
| US | 4 × 12 ft | 48 ft² | Fewer joints on long walls |
| UK | 2400 × 1200 mm | 2.88 m² | Standard full board |
| UK | 1800 × 900 mm | 1.62 m² | One-person handling, ceilings |
Don’t forget
- Moisture-resistant board (green board / MR plasterboard) for bathrooms and kitchens, or cement board behind tiles in showers.
- Fixings and finishing: screws, joint tape and compound (US), or scrim tape and finishing plaster or jointing compound (UK).
- After finishing, the paint calculator works out primer or mist coat and topcoats.
Frequently asked questions
How many sheets of drywall do I need for a 12 × 12 room?
With 8 ft walls, the four walls are 384 ft². Using 4 × 8 ft sheets with 10% extra, you need 14 sheets for the walls, or 19 sheets including the ceiling. Larger 4 × 12 sheets reduce the count and the number of joints.
What size is a sheet of plasterboard in the UK?
The standard full sheet is 2400 × 1200 mm (2.88 m²). Smaller 1800 × 900 mm and 2400 × 900 mm boards are easier for one person to handle and fit into cars, and are common for ceilings and small rooms.
What thickness of drywall or plasterboard should I use?
In the US, 1/2 inch drywall is standard for walls, and 5/8 inch is often used on ceilings and where fire-rated (Type X) board is required. In the UK, 12.5 mm board is standard for walls and ceilings, with 15 mm used for better sound or fire performance. Building codes and regulations may set specific requirements.
Should I subtract doors and windows?
For accurate ordering, yes — the calculator removes a typical area for each. Many installers still board over small openings and cut them out afterwards, which is why a waste allowance of around 10% is included.
How do I reduce waste and joints?
Use the longest sheets you can handle and fit through the door, so walls need fewer horizontal and end joints. Hanging sheets horizontally on walls is common in the US because it reduces the length of seams to tape.