I was helping a neighbour board out a bedroom conversion a while back. He had calculated the boards carefully - or so he thought. He had measured all four walls, added them up, divided by the board area and come up with a number he was satisfied with. What he had not done was account for the door and window openings, and he had not added any wastage at all.
We were three boards short with two walls left to do. At 6pm on a Saturday with the merchant closed. That particular bedroom did not get finished until the following weekend.
The calculation is not complicated - it just needs to be done properly.
Standard board sizes
The overwhelming majority of domestic plasterboarding uses the standard 2400 x 1200mm (8ft x 4ft) board. Here are the sizes you will typically encounter:
| Size | Area | Common use |
|---|---|---|
| 2400 x 1200mm | 2.88 m2 | Walls - the standard |
| 2400 x 900mm | 2.16 m2 | Ceilings, narrow walls |
| 2700 x 1200mm | 3.24 m2 | Rooms with higher ceilings |
| 3000 x 1200mm | 3.6 m2 | High ceilings, fewer joints |
| 1200 x 900mm | 1.08 m2 | Small areas and patching |
Thickness: Standard walls use 12.5mm. Ceilings often use 9.5mm to keep the weight lower - holding heavy boards flat overhead while screwing them is hard enough without the extra weight.
How to calculate boards for walls
Step 1: Calculate gross wall area
Measure each wall: height x width. Add all walls together.
Step 2: Deduct openings
Measure each door and window opening. Subtract the area of each from the gross total.
Step 3: Add wastage
Add 10% for a simple rectangular room.
Step 4: Divide by board area
Divide by 2.88 m2 for standard 2400x1200mm boards. Round up to whole boards.
Worked example - living room
Room 5m x 4m, ceiling height 2.4m:
Gross wall area:
- Two 5m walls: 2 x (5 x 2.4) = 24.0 m2
- Two 4m walls: 2 x (4 x 2.4) = 19.2 m2
- Total: 43.2 m2
Deductions:
- One door (2.1 x 0.9m) = 1.89 m2
- Two windows (1.2 x 1.0m each) = 2.4 m2
- Total deductions: 4.29 m2
Net wall area: 43.2 - 4.29 = 38.91 m2
Add 10% wastage: 38.91 x 1.1 = 42.8 m2
Boards needed: 42.8 / 2.88 = 14.9, round up to 15 boards
Ceiling calculation - same room
- Ceiling area = 5 x 4 = 20 m2
- Add 10% wastage = 22 m2
- Boards (2400x1200mm): 22 / 2.88 = 7.6, round up to 8 boards
Total for the room: 23 boards (15 walls + 8 ceiling).
Use the Plasterboard Calculator to do this calculation for multiple rooms at once.
Quick reference: boards by common room size
Standard 2400x1200mm boards, walls only, 10% wastage, 2.4m ceiling height, one door, two windows:
| Room size | Net wall area | Boards for walls | Boards for ceiling |
|---|---|---|---|
| 3m x 3m | approx 24m2 | 9 | 4 |
| 4m x 3m | approx 29m2 | 12 | 5 |
| 4m x 4m | approx 33m2 | 13 | 6 |
| 5m x 4m | approx 39m2 | 15 | 8 |
| 5m x 5m | approx 44m2 | 17 | 9 |
| 6m x 4m | approx 44m2 | 17 | 10 |
Deductions and wastage are estimates. Calculate your exact room with the calculator.
Fixings: how many screws?
For plasterboard fixed to timber stud walls:
- Use 3.5mm drywall screws, 38mm or 42mm long for 12.5mm board
- Space at 300mm centres on intermediate studs, 150mm on edges and perimeter
Approximate screws per board: 28-32 for a standard 2400x1200mm wall board.
| Boards needed | Approximate drywall screws |
|---|---|
| 10 boards | 300 screws |
| 15 boards | 450 screws |
| 20 boards | 600 screws |
| 30 boards | 900 screws |
Buy them in boxes of 200 or 500 - they are cheap and you will always have leftovers that come in useful.
Jointing tape and compound
Every board joint needs jointing tape and compound:
- Joint depth tape (paper or fibreglass mesh): budget roughly 1m of tape per running metre of joint
- Jointing compound / filler: approximately 1.5-2kg per running metre of joint
The number of joints depends on board arrangement. For a standard stud wall boarded with 2400x1200mm boards at 2.4m ceiling height, you will have vertical joints every 1.2m across the width of the wall.
My tips for plasterboarding
After boarding out several rooms in our house, including a full loft conversion, here is what I have learned:
Work out stud centres before you start buying boards. If your studs are at irregular centres, or wider than 600mm, your board joints may not fall on a stud. Plan board layout on paper first. A board joint not on a stud will crack over time.
Mark stud positions on the floor before boarding. Pencil lines on the floor at each stud location mean you can find them easily after the boards are up. Trying to find studs through plasterboard by trial and error (or worse, by magnetic stud finder) wastes time and leaves unnecessary holes.
Use a deadman prop for ceiling boards. A T-shaped prop made from a length of timber with a crossbar holds a ceiling board flat against the joists while you drive screws. Without one, ceiling boarding is a two-person job and a backbreaking one at that. With one, it is perfectly manageable alone.
Board the ceiling before the walls. Standard practice - the ceiling boards can be supported by the walls on completion, and the wall boards can butt up snugly under the ceiling edge.
Stagger vertical joints. Do not line up all the vertical joints on adjacent walls - offset them so they are not in the same plane. This improves the structural integrity and reduces cracking.
Get your board count and fixings sorted with the Plasterboard Calculator before your next trip to the merchant.