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How the plasterboard calculator works
The calculator treats the room as a rectangle. You give it the length, the width and the wall height, and it works out the length of wall you are actually boarding: all four walls is 2 × (length + width), two long walls is 2 × length, one wall is simply the length. Multiply that run by the wall height and you have the wall area in square metres. Tick the ceiling box and it adds length × width for that as well.
That gross area is then increased by your waste allowance (10% by default) and divided by the area of one board. A standard 2400 × 1200 mm sheet is 2.88 m², so a 4 m × 3 m room with 2.4 m walls and a ceiling comes out at 45.6 m² before waste and 50.16 m² after — 18 sheets. The division is always rounded up, because you cannot buy part of a sheet.
Choosing the right board
Most UK walls take 12.5 mm tapered-edge board on 400 mm stud centres. 9.5 mm is lighter and cheaper and is the usual choice for overboarding an existing ceiling or walls, because it adds less weight. 15 mm is worth the extra money where sound transfer matters, such as a party wall or a bedroom wall shared with a bathroom.
Board length matters more than people expect. A 2400 mm board leaves a horizontal joint on a 2.4 m wall; a 2700 mm board covers standard 2.4–2.6 m ceilings in one piece with less taping. Longer boards are heavier and awkward to carry up stairs, so weigh up access before ordering 3000 mm sheets.
Why the 10% waste allowance
Plasterboard is cut around sockets, door frames, radiators and pipework, and offcuts rarely fit anywhere else. Allow 10% for a simple square room, and nearer 15% for a room with lots of openings or a diagonal ceiling. Keep one whole sheet back if you can — it is far cheaper than buying a single replacement later if a sheet gets damaged.
What else you will need
Screws go in at 400 mm centres, which works out at roughly 30 screws per square metre of board. Scrim tape for the joints and internal corners runs at roughly 1.5 m per square metre; it is sold in 90 m rolls. Then joint filler for taping and filling, and longer screws if you are overboarding (around 50 mm, so they bite into the studs rather than the old board).
Plasterboard FAQs
How many sheets of plasterboard do I need for a 4 m × 3 m room?
With 2.4 m walls, all four walls and the ceiling, you need 18 sheets of 2400 × 1200 mm board. That is 45.6 m² of surface, plus 10% for cuts (50.16 m²), divided by 2.88 m² per sheet and rounded up. Walls only — no ceiling — is 33.6 m², or 13 sheets with the same allowance.
Should I use 9.5 mm or 12.5 mm plasterboard?
12.5 mm is the default for new walls and overboarding ceilings in most UK houses; it feels more solid and gives better sound insulation. 9.5 mm is fine for overboarding walls and ceilings where weight and cost matter, but it dents more easily on a wall that gets knocked.
How much waste should I allow?
10% covers a straightforward room. Go to 15% if there are several doorways, sockets or pipe runs to cut around, or if you are working alone and likely to break the occasional sheet. Waste allowance is insurance, not optimism.
Can I plasterboard over existing plasterboard?
Yes — it is called overboarding, and it is often quicker and cleaner than stripping the old wall back to the studs. Use 9.5 mm board and screws long enough to reach the studs (typically 50 mm), and mark the stud positions first so every fixing lands on timber.
Can I tile straight onto plasterboard?
Not in wet areas. Bathrooms and shower walls need moisture-resistant board, and the board must be a suitable substrate for the tile weight. In showers and behind baths, use a tile backer board instead. Check the tile adhesive manufacturer's guidance before committing.