Plasterboard calculator sheets, screws and tape

How the calculator counts the sheets

It takes your area, subtracts the openings, adds 10% for cuts, and divides by the area of one board. Then it works out the fixings and finishing from the same boarded area, using the manufacturers' quantities per square metre. Everything is rounded up to whole sheets, rolls and bags.

The screw count depends on whether you are boarding walls or a ceiling, because ceilings are fixed at closer centres. The finish is your choice: Tape and joint for a dry lined finish, or Skim coat for a plastered one. Tape and joint shows the tape rolls and bags of joint cement. Skim coat shows the bags of finish plaster.

The calculator opens on this example: 30m² of walls with one door opening of 2.0m × 0.9m, 2400 × 1200mm boards and the fixed 10% allowance for cuts.

• Area to board: 30 − 1.8 = 28.2m². With 10% wastage, 28.2 × 1.10 = 31.02m².
• Boards: 31.02 ÷ 2.88 (one 2400 × 1200 sheet) = 10.8, so 11 boards.
• Screws: 31.02 × 11 per m² = 341.2, so 342 screws.
• Joint tape: 31.02 × 1.5m per m² = 46.5m, so 1 roll of 150m.
• Tape and joint: 31.02 ÷ 55m² per 25kg bag of joint cement = 0.56, so 1 bag.
• Or tape and skim: 31.02 ÷ 10m² per 25kg bag at 2mm = 3.1, so 4 bags of finish plaster.

Which board size to buy

Standard 12.5mm board comes in 900mm and 1200mm widths, in lengths from 1.8m to 3.0m, with tapered edges for jointing or square edges for skimming. Those ranges are from Siniat's drywall manual, which also gives 12.5mm board as 8.0kg per m². The width you choose sets the stud spacing behind it: British Gypsum's Site Book gives 600mm stud centres for 1200mm wide WallBoard and 450mm for 900mm wide, in both 12.5mm and 15mm.

Screw centres and screw counts

British Gypsum's Site Book fixes boards to timber studs with screws at 300mm maximum centres, closing up to 200mm at external corners. On timber joist ceilings the screws go at 230mm centres. Keep each screw at least 10mm from a bound (paper-covered) edge and 13mm from a cut edge, and use a length that goes about 25mm into the timber.

The calculator uses British Gypsum's own take-off quantities, which include the perimeter fixings: 1,100 screws per 100m² for its single layer GypLyner wall lining and 1,560 per 100m² for a single layer ceiling fixed direct to timber joists. That is 11 per m² on walls and 15.6 per m² on ceilings. The wall figure comes from a metal-framed lining, but the timber stud partition take-off is close: its face layer works out at 11.25 per m². The tool uses these rates as fixed figures.

For the length, the same book gives 32mm Drywall Timber Screws for 9.5mm board, 38mm for 12.5mm and 15mm board, and 41mm for 19mm plank. Two layers need longer screws: 51mm for 12.5 over 12.5mm, and 60mm for 15 over 15mm.

Tape and joint or tape and skim

Both finishes start with tape over the board joints. British Gypsum's jointing take-off allows 150m of joint tape per 100m² of board, which the calculator uses as 1.5m per m². Both British Gypsum and Siniat sell joint tape in 150m rolls. The calculator only shows the tape when you pick tape and joint, so allow for it yourself if you skim.

Tape and joint fills the tapered edges with jointing compound in two or three coats, sanded between, then the whole board is primed before painting. Siniat gives 55m² per 25kg bag of its Joint Cement and 50m² per 20kg tub of its ready-mixed Smartmix, each for a full taping operation. The calculator uses the bag figure. If you use tubs or another product, divide the boarded area by that product's coverage instead.

Tape and skim (the Skim coat option) puts a 2mm coat of finish plaster over the whole board. Thistle Board Finish and Multi-Finish cover 10m² per 25kg bag at 2mm, the figure British Gypsum gives in the Site Book. For a different thickness, use the plaster calculator.

Worked example: a ceiling on your own

Say you are boarding a living room ceiling 4.2m by 3.3m, which is 13.86m². Choose A ceiling, enter 13.86 as the area and 0 for doors and windows, then compare two board sizes:

• Area with 10% for cuts: 13.86 × 1.10 = 15.25m².
• 2400 × 1200 boards: 15.25 ÷ 2.88 = 5.3, so 6 boards at about 23kg each.
• 1800 × 900 boards: 15.25 ÷ 1.62 = 9.4, so 10 boards at about 13kg each. Four more sheets, but each one can be held up by one person.
• Screws at ceiling centres: 15.25 × 15.6 = 237.8, so 238 screws. The same area on a wall would take 168.
• Joint tape: 15.25 × 1.5 = 22.9m, so 1 roll. Joint cement: 15.25 ÷ 55 = 0.28, so 1 bag. Or skim: 15.25 ÷ 10 = 1.5, so 2 bags.
• Fix the boards with their long edges across the joists and cut ends landing on a joist or a nogging, as British Gypsum's ceiling instructions set out.

Before you order the boards

The 10% wastage is our working allowance, not a maker's figure. British Gypsum's and Siniat's quantities make no allowance for waste, cut-outs or damaged sheets. The tool fixes it at 10%, so on a room with lots of short returns, boxing-in or a sloping ceiling, add a few spare sheets.

Take off big openings such as doors, patio doors and windows. Leave small ones like sockets and switches in, because the board around them is still a whole sheet. If a window opening has deep reveals, add the reveal faces back on as extra area.

The calculator covers a single layer of standard 12.5mm board fixed to timber. It does not count corner or angle bead, noggings, adhesive for dot and dab, primer, or the extra screws for double layers. Fire, acoustic and moisture resistant boards may need different fixings and centres, so check the system you are building to. Where a wall or ceiling has to meet a fire or sound rating, the board and fixing details come from that system's specification, not from a general calculator.

Pricing the job? Put the sheet and bag counts on the quote, and work out what your time is worth with the day rate calculator. When the walls are done, the paint calculator does the next step.

Common questions