Plasterboard is sold in whole sheets of a fixed size, and a room is a set of rectangles that do not divide by it. A wall 5.4 m long and 2.4 m high takes three widths of board across and two heights up, which is six sheets for 12.96 square metres of wall — and that is the useful thing to know, because it means the sheet count is driven by the layout of the wall, not by its area. Two rooms of the same area can need different numbers of sheets. This sets the board out on each wall and counts what you actually buy.
Walls, sheet size and waste
Sheets are 2.88 m² and walls are rarely a multiple of that.
Ten percent is normal; fifteen if there are many openings.
Result
Sheet count follows layout, not area
This is the thing worth knowing about plasterboard: it is a rectangle being fitted to a rectangle, and the number you need is decided by how the two rectangles relate. A wall 5.4 m long takes three widths of 1.2 m board across, whatever its height. A wall 2.4 m high takes one height of 2.4 m board, or one and a bit of a shorter sheet. Multiply and you have the sheet count for that wall — and no amount of area arithmetic will give you the same answer.
The consequence is that two rooms of identical area can need different numbers of sheets. A room 6 m by 3.6 m and a room 4.8 m by 4.5 m have the same area, but the first takes five widths along the long wall and the second four, and the offcuts fall differently. It is worth setting each wall out individually rather than multiplying a total area by a coverage figure.
Where the waste actually comes from
Plasterboard waste is not offcuts in the usual sense, because offcuts are usable: a piece left over from one wall often fits another. The real waste is at openings, where a sheet has to be cut around a door or a window and the piece above the opening is a specific shape that fits nowhere else; and it is in the tapered edges, because a cut edge is not a tapered edge and two cut edges meeting do not tape invisibly.
That second point drives where you cut rather than how much you waste. Board is manufactured with tapered long edges for jointing, and the cut ends are square. Joints between tapered edges disappear under tape; joints involving a cut end need more filler and more sanding and are more likely to show. Setting the layout so that the cut ends fall at the floor and the ceiling, and the tapered edges meet in the middle of the wall, saves more finishing time than any amount of careful cutting.
A 1200 x 2400 mm sheet of 12.5 mm board weighs about 25 kg, and it is brittle at the corners. Carry it on its edge, not flat, and never let a corner take a knock.
Two people for a full sheet, and a board lifter for the ceiling. A sheet dropped on a corner is a sheet with a hole in it.
Choosing the board
Standard 12.5 mm board is the default for walls and 9.5 mm for overboarding. Moisture-resistant board — green — for bathrooms and kitchens. Fire-rated board — pink — wherever a fire rating is specified, which on a domestic job usually means around a garage, a boiler or a staircase. Acoustic board, which is denser and laminated, where sound matters.
Bigger sheets are not automatically better. A 1200 x 3000 mm sheet covers a 2.7 m room in one height with no horizontal joint, which is a real saving in finishing; but it weighs over 30 kg, it will not go up a tight stair, and it is much harder to handle in a small room. The narrow 900 mm board is worth considering for one-person work.
| Room | Sheet size | Waste | Type | Note |
|---|---|---|---|---|
| Living room, standard | 1200 x 2400 | 10% | Standard | The default case |
| Bathroom | 1200 x 2400 | 15% | Moisture-resistant | Green board, and more cuts |
| Room at 2.7 m | 1200 x 3000 | 8% | Standard | No horizontal joint |
| Garage or boiler | 1200 x 2400 | 12% | Fire-rated | Pink; check the spec |
| Bedroom, for sound | 1200 x 2400 | 12% | Acoustic | Denser, heavier |
| Overboarding | 1200 x 2400 | 8% | 9.5 mm | Over existing plaster |
A worked example
- One wall5.4 m across takes 3 lengths at 2.4 m; 2.4 m up takes 2 courses of 1.2 m. Six sheets per wall.
- Four wallsThat is 24 sheets before waste, covering 51.84 m².
- AllowanceTen percent takes it to 27 sheets.
- CostAt $18 a sheet, $486.
- Check the coverageTwenty-seven sheets is 77.8 m² of board for 51.84 m² of wall — about 26 m² of offcuts, some of which will be usable.
Twenty-six square metres of offcuts sounds alarming and is not, because a large share of it is usable: a 1.2 m drop from one wall fits the next wall's short end. The genuine waste is the shaped pieces around openings and anything under about 300 mm.
Fixing and finishing
Fix board horizontally on walls where you can, because it puts the tapered joints at a comfortable height and reduces the length of joint you have to tape. On ceilings, board across the shorter dimension and stagger the end joints. Leave a 3 mm gap at the floor — board should not touch the floor, or it wicks moisture.
Screw it, do not nail it: screws at 300 mm centres on the field and 200 mm on the edges, driven just below the surface without breaking the paper. Broken paper at a screw is a pop waiting to happen, and pops are the most common complaint about a skimmed wall twelve months later.
Then the jointing, which is where the material estimate continues: the joint compound figure covers the filler, and the wall area figure gives the surface for painting once it is finished. If the studs behind the board are being counted too, the stud figure is the one that sets the centres the sheets land on.
Do
- Set the board out wall by wall, not by total area.
- Put tapered edges together and cut ends at floor and ceiling.
- Use green board in wet rooms and pink where a fire rating is specified.
- Leave a small gap at the floor so it cannot wick moisture.
- Screw at 300 mm centres without breaking the paper.
Avoid
- Counting sheets from the area and running short.
- Putting two cut ends together, which tapes visibly.
- Letting board touch the floor.
- Nailing rather than screwing, which pops later.
- Carrying a full sheet flat, which is how corners break.
Where the numbers stop being true
The layout assumes plain rectangular walls with no openings. Every opening adds cuts and reduces the usable offcuts, and a room with three openings and a chimney breast wastes considerably more than ten percent. Raise the allowance, or better, set the board out on paper first.
Ceilings are not covered here, and they behave differently: they are usually boarded across the joists with the tapered edges on the long dimension, and the waste is lower because the shape is simpler. The ceiling area figure is the right starting point for that surface.
Nothing here accounts for access. A sheet that will not go round the bend in the stair has to be cut, and a 3 m sheet in a terrace house is frequently a 3 m sheet that does not get in. Check the route before ordering the bigger size.
How the figures are produced
Sheets across is the wall length divided by 1.2 metres, rounded up. Sheets up is the wall height divided by the sheet height, rounded up. Their product is the sheets per wall, multiplied by the number of walls to give the count before waste. The waste percentage is applied and the result rounded up to whole sheets. The stack compares what the sheets cover with the wall area, so the offcut share is visible.
Questions people ask
How many sheets for a 5.4 x 2.4 m wall?
Six: three lengths of 2.4 m board hung horizontally, and two courses of 1.2 m up.
Why not just divide the area by 2.88?
Because sheets are rectangular and so are walls. The count depends on how the two rectangles relate, and area arithmetic gives a different — usually lower — answer.
How much waste should I allow?
Ten percent for plain walls, fifteen with several openings, eight if the offcuts cascade down a run of similar walls.
Should I board horizontally or vertically?
Horizontally on walls: fewer joints, and the tapered edges land at a comfortable working height.
Can I use offcuts?
Yes, for anything over about 300 mm, as long as the joint lands on a stud or a noggin and at least one edge is tapered.
Green or standard board in a bathroom?
Green — moisture-resistant — in any wet room, and consider a cement-based board behind a shower rather than either.