Plasterboard Calculator UK · metric

Boards, screws, jointing compound and scrim for walls and ceilings. Metric units and £ costs, updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Boards = area ÷ board size, plus 10% for waste and cuts. Standard boards are 2.4 × 1.2 m (2.88 m²). Estimates add plasterboard screws (~12/m²), jointing compound (~0.6 kg/m²) and scrim/joint tape.

The waste allowance

This calculator adds 10% to the area before dividing it into boards. Nearly all of that allowance ends up as offcut you will never use again: the narrow strip beside a door reveal, the corner piece too short to land on two studs, the board that takes a knock coming up a stairwell. It is also why the calculator does not ask you to deduct windows and doors. The board cut out of an opening is mostly scrap, so subtracting the openings and then adding waste on top leaves you short. Running one board light at four in the afternoon means another trip to the merchant, and usually a joint in the middle of a wall that did not need to be there.

Board sizes and what they weigh

The standard British plasterboard is 2400 x 1200 mm, which is 2.88 m² of cover, and the handy size is 1800 x 900 mm at 1.62 m². British Gypsum also list Gyproc WallBoard at 1200 x 2700 and 1200 x 3000 mm, so a 2.7 m ceiling can be reached in one piece rather than two with a joint across the middle.

Thickness is normally 9.5 mm or 12.5 mm for domestic work, with 15 mm where fire performance or acoustic mass is wanted. Twelve and a half millimetre wallboard is the default for walls and ceilings in a modern house. The 9.5 mm board is lighter and cheaper but wants closer support, so it belongs on tightly spaced ceiling joists or a wall lining rather than as a general substitute.

Weight decides how you work more than cost does. British Gypsum give the nominal weight of 12.5 mm Gyproc WallBoard as 7.7 kg per square metre, putting a full 2400 x 1200 board a little over 22 kg. That is liftable by one person on a wall and awkward on a ceiling. Longer boards cut the number of joints to finish, which is the real prize, but a 3000 x 1200 sheet is nearer 28 kg and needs two people and a room that will take it round the corner.

Tapered edge for jointing, square edge for skim

This is where British practice parts company with American. In the United States board is taped and filled and the compound feathered out over the seams, with the rest of the face left as paper. Here the normal domestic finish is a skim: the whole surface takes a thin coat of finishing plaster, joints and board face alike, trowelled up to a continuous polished surface ready for paint. The seam problem largely disappears because no seam is left visible to hide.

The board you buy follows the finish you intend. Tapered edge board has a shallow recess along its long edges so tape and jointing compound sit flush with the face, and it is the board for a taped and jointed dry lining. Square edge has no recess and is meant to be skimmed, because the plaster covers everything anyway. Tapered edge that gets skimmed is common and harmless. Square edge that someone then tries to tape is not, because there is no recess for the tape and every joint finishes proud.

The two routes consume quite different materials. Thistle MultiFinish, the usual skim plaster, is quoted at roughly 10 m² per 25 kg bag at 2 mm with a setting time of about 90 minutes, and it goes on in two passes over the whole surface. Jointing uses far less material over a much smaller area: British Gypsum's data for Gyproc Easi-Fill puts the taping coat on flat tapered edge joints at about 18 kg per 100 linear metres, mixed at roughly 7 litres of water to a 10 kg bag, with the joint finished in two applications. Skimming needs a plasterer. Jointing is slower but far more forgiving of an amateur, which is why it is the sensible DIY route.

Fixing is a spacing rule in millimetres

There is no British figure for screws per board, because the manufacturers publish spacings rather than counts. British Gypsum give 300 mm centres on partitions, tightening to 200 mm at external angles, 230 mm within the field of a ceiling with 150 mm at board ends, and 150 mm throughout on encasements. A screw count only falls out of those figures once you know your stud or joist layout.

Work it through and the numbers are less mysterious than they look. Studs at 600 mm centres with screws at 300 mm gives around six fixings per square metre in the field of a wall, before the closer spacing at the perimeter and at external angles. A ceiling on joists at 400 mm centres with screws at 230 mm is nearer eleven, and the 150 mm at board ends pushes it higher. About twelve per square metre is generous for walls and roughly right for ceilings, which is why it is safe to order against.

One rule holds regardless of spacing. British Gypsum's data sheet for 12.5 mm WallBoard asks for fixings at least 13 mm from cut edges and 10 mm from bound edges. Closer and the head breaks through the paper liner, which is the layer that actually carries the load, so the fixing holds very little and shows through the finish later. The same sheet asks you to stagger joints between layers by at least 600 mm where you are boarding in two layers.

Where the regulations choose the board for you

Most of a house is a free choice between 9.5 mm and 12.5 mm board, decided by joist spacing and how much you care about sound. Some of it is not. Approved Document B sets fire resistance periods for particular elements, and the board specification is how you meet them. An enclosure to a protected stairway, for example, is commonly built as EI 30 using a single layer of 15 mm Gyproc FireLine each side of a 70 mm stud. Standard wallboard is not the same product and does not carry the same tested performance.

Approved Document B does not name manufacturers' products, and that is the trap. It sets the performance and you demonstrate compliance with a tested system: the right board, layers, stud and fixings as a package, not a thickness picked off a shelf. British Gypsum's White Book and Knauf's technical manual set out the tested build-ups, and building control will want to know which you used.

The cases that catch people out are the ceiling under a loft conversion, the wall and ceiling between an integral garage and the house, and the underside of a stair that has become part of an escape route. We are not going to quote a thickness for those here, because the answer depends on storey height, layout and whether the element is a compartment wall or an enclosure. Taking it from a calculator page rather than building control is how people end up pulling a finished ceiling down again.

Screw fixing centres and edge distances

Fixing centres for plasterboard, as published by British Gypsum. Screws per board are derived from these spacings and your own stud or joist layout, not published as a flat figure.

Screw fixing centres from British Gypsum's published guidance on fixing centres for partitions, ceilings and encasements. Edge distances, board sizes and the 7.7 kg/m² nominal weight from the British Gypsum product data sheet for Gyproc WallBoard 12.5 mm. Skim coverage from the Thistle MultiFinish product listing and jointing coverage from the Gyproc Easi-Fill data sheet.

What goes wrong boarding out

Square edge board on a taped and jointed job. Square edge has no recess for the tape, so every joint finishes proud of the board face. Square edge is for skimming, tapered edge is for jointing, and mixing them up is the most common ordering error.

Screws driven too close to the edge. British Gypsum ask for at least 13 mm from a cut edge and 10 mm from a bound edge. Closer and the head tears the paper liner, leaving a fixing that holds almost nothing.

Using wall spacing on a ceiling. Ceilings are fixed at 230 mm in the field and 150 mm at board ends because the board carries its own weight the whole time. Three hundred millimetre centres overhead is how ceilings start to sag.

Assuming standard wallboard will satisfy Part B. Fire resistance is demonstrated with a tested system, not a board thickness. Where a period of fire resistance applies, the board type, layers, stud and fixings are all part of the specification.

Ordering plaster to the exact area. Ten square metres per 25 kg bag of MultiFinish assumes an even 2 mm. Suction and a rough background push that up, and the plaster has a 90 minute set, so running out mid wall is not something you can pause for.

Frequently asked questions