The job
Modelled, not recorded. Every figure here is the answer the Drywall Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The room is a back bedroom 4.2 m (13.78 ft) by 3.3 m (10.83 ft), with walls 2.4 m (7.87 ft) high, one internal door and one wide window. The old plaster has come off, as it does when it has blown or been chased out for a rewire, and the walls are being reboarded rather than replastered from scratch. The ceiling stays, so this count is walls only.
The openings are entered as the sizes the room actually has rather than left at the calculator's defaults. A UK internal door leaf with its lining takes 1.66 m² off the wall, and the window, a wide casement over a radiator, takes 1.8 m². These are worth measuring because a window this size is close to the area of a whole board, and the calculator deducts exactly what it is given.
Two of the room's walls are solid masonry and two are timber stud partitions. That split matters for the fixings and not at all for the board count: the calculator works from area, so the walls are the same to it whichever way the boards go up. It counts the standard metric board, the sheet a UK merchant stacks by default.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Walls to Board | Four walls of a rectangular room | Left at the calculator's own default. |
| Room Length | 4.2 m (13.78 ft) | Taken at skirting height, wall face to wall face, after the old plaster is off: the bare wall is what the boards cover. |
| Room Width | 3.3 m (10.83 ft) | Measured the same way; in an older house check it at both ends, because a back addition is rarely square to the main house. |
| Wall Height | 2.4 m (7.87 ft) | Left at the calculator's own default. |
| Number of Doors | 1 | Left at the calculator's own default. |
| Number of Windows | 1 | One window in the walls being boarded; the reveals are boarded later as narrow strips and are not part of this deduction. |
| Area of One Door | 1.66 m² (17.87 sq ft) | A UK internal door leaf with its lining, measured rather than left at the calculator's default, which is a larger opening. |
| Area of One Window | 1.8 m² (19.38 sq ft) | The structural opening of the window, measured across the brickwork, not the glass. |
| Waste Factor (%) | 10 | Left at the calculator's own default. |
The working, step by step
1. Gross wall area
36 m² (387.5 sq ft)
Twice the sum of length and width, times the height, gives 36 m². The calculator assumes every wall rises to the same height and that the room closes on itself; a chimney breast is extra wall the formula does not see, and in a house of this age it is worth adding by measuring its two returns and entering a slightly longer room.
2. Doors + windows area (subtracted)
3.46 m² (37.24 sq ft)
The door and the window together come to 3.46 m², and the calculator takes all of it off before anything else happens. On this room the deduction is large for one door and one window because the window is wide; it is the reason the opening sizes were measured instead of trusted to the defaults.
3. Net wall area
32.54 m² (350.3 sq ft)
The wall left to cover is 32.54 m². This is the figure to check against a quick count on site, wall by wall, before buying: an error here is an error in the tape, and every later row inherits it.
4. Area with waste factor
35.79 m² (385.3 sq ft)
The calculator's 10 per cent goes on top, to 35.79 m², which is then divided by the area of one board and rounded up. The allowance has to cover the narrow strips beside each opening and over the door head, which are where most of a board's offcut ends up in a room like this.
5. Joint tape needed
39.5 m (129.6 linear ft)
Tape is taken as a fixed rate per area of board, 39.5 m here. In a UK room the boards are usually given a thin skim of finishing plaster rather than taped and filled, and the joints under a skim are covered with scrim tape; the row still measures the joints, but the jointing compound the US pages count is replaced by the skim.
6. Drywall screws needed
416 screws
The screw count, 416 screws, is a fixed number per board, worked out for boards screwed to framing. It fits the two stud partitions and overstates the job for the masonry walls, where boards are normally bonded with dabs of adhesive and need only a few fixings each.
Estimated drywall needed
13 sheets (2400x1200 mm)
The order is 13 sheets (2400x1200 mm), for the walls only. The calculator counts the standard metric board for a metric reader; the same room counted for a US reader comes to 13 sheets (4x8 ft), a different board, so the count never travels without its board size. Boards come in more than one length and thickness, and a merchant will ask for both, so read the thickness from the job's specification and take the length from the wall height, which is the one decision this calculator leaves entirely to you.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Estimated drywall needed | 13 sheets (2400x1200 mm) | Standard metric boards for the walls; the thickness and edge type come from the specification, not from this count. |
| Joint tape needed | 39.5 m (129.6 linear ft) | Scrim or paper tape for the joints, depending on whether the walls are skimmed or taped and filled. |
| Drywall screws needed | 416 screws | Screws for the boards on the stud partitions; on the masonry walls the fixing is adhesive, counted separately. |
The allowance, and what it does here
The 10 per cent is added to the net wall area and then the board count is rounded up, so the last board carries some of the margin already. In a room with one large opening most of the waste is in the strips around it: a board cut to fit beside a window leaves an offcut that is only useful if another strip of the same height turns up, and in a small room it rarely does. A room with a chimney breast, a boxed-in pipe and two alcoves is the kind that wants the larger allowance the calculator's guidance mentions.
The common mistake
Buying screws for walls that will be bonded
The calculator's screw row assumes every board is screwed to framing, which is how a US wall is built. A UK bedroom often has solid masonry outside walls where the boards are stuck on with dabs of adhesive and a handful of fixings, so a screw order taken straight from the row is several boxes too many, while the adhesive, which the row does not count at all, is forgotten. Split the walls by how they will be fixed, and count the adhesive for the masonry walls with its own calculator before going to the merchant.
Change the job, and see what moves
The same room, with a period ceiling height
Wall Height: 2.7 m (8.86 ft) → 15 sheets (2400x1200 mm)
Put the same room in a period house with walls 2.7 m high and the count becomes 15 sheets (2400x1200 mm). The area rises in proportion to the height, but the calculator does not know that a standard board is now shorter than the wall: hung upright, each one needs a strip above it and a horizontal joint, and the cleaner answer is a longer board, which a merchant stocks in fewer thicknesses. Decide the board length before the count is final.
The same room, with its window bricked up
Number of Windows: 0 → 14 sheets (2400x1200 mm)
Take the window out of the job, as when it is bricked up during the work, and the count rises to 14 sheets (2400x1200 mm). A wide window really is worth a whole board on a room this size, which is the one case where an opening deduction changes what is bought rather than just the offcuts; on a room with a small window the deduction often disappears into the round-up.
What to run next
- Dot-and-Dab (Direct Bond) Drywall Adhesive Calculator
The masonry walls are bonded, not screwed, and the adhesive is counted from the board area those walls take.
- Skim Coat Plaster Calculator
A UK board is usually finished with a thin skim of plaster, and the bags of finish are counted from the same wall area.
- Plaster Angle and Stop Bead Calculator
The window reveals and any external corners are finished with bead, measured by length around the same openings.
Sources
The method and every constant behind these figures belong to the Drywall Calculator, version 1.2.0, last verified 2026-09-17. It rests on:
- Standard drywall sheet size is 4 ft x 8 ft (32 sq ft / 2.973 m²) in North America and 2400 x 1200 mm (2.88 m²) in metric markets — the most common sizes sold for wall installation
- 10-15% waste factor is standard industry practice for rooms with doors, windows, and outlet cutouts
- Joint tape: approximately 370 linear ft per 1,000 sq ft of drywall area (common manufacturer/retail estimating rule); screws: approximately 32 per standard sheet — 4x8 ft or 2400x1200 mm, within 3% of each other in area — on 16 in o.c. framing
Frequently asked questions
- How many plasterboard sheets for a 4.2 by 3.3 m room?
- With walls 2.4 m high, one door and one wide window, the walls take 13 sheets (2400x1200 mm), from a net wall area of 32.54 m² with the calculator's 10 per cent allowance. The ceiling is not included.
- Does the window really make a difference to the board count?
- Here it does: without the window the same room needs 14 sheets (2400x1200 mm). The opening takes 3.46 m² off the walls together with the door, enough to cross a board boundary.
- Why does the calculator give a different count for a US reader?
- Because it counts the board each market sells by default. For a US reader this room is 13 sheets (4x8 ft); the area is the same, the board is not, so a count is only meaningful beside its board size.