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Spec Check
Material notes
Calculation basis
What this page assumes before you type anything, and how the answer is rounded. Every value below is this calculator's own default: change the box and the answer changes with it.
- Board length
- 2400 mm (standard)this page's own default
- Board width
- 1200 mm (standard)
- Board thickness
- 12.5 mm (standard walls)
- Waste allowance
- 10%
- Rounding
- Up to the next whole unitsheets, screws and scrim-tape rolls all round up
- Last reviewed
- 25 September 2026checked against this page's own defaults
Confirm quantities against your actual product and site conditions before ordering — standard sizes vary between suppliers, and the ground the material lands on is yours, not ours. Figures are planning estimates, not a quote.
Don't forget — check before you order
On this boarded room the calculator returns these as estimates or optional extras rather than a fixed quantity, so they are listed here instead of being counted into the shopping list. Each one is the calculator's own wording.
- Corner bead (external corners) — Rule of thumb: measured along the external corners only — an internal corner is taped and filled, not beaded. Supplied in 2.4 m or 3 m lengths, so count the corners and round up.
- Plasterboard adhesive (dot-and-dab onto masonry) — Rule of thumb: only where boards are dabbed onto a masonry wall rather than screwed to a stud or joist. Roughly one 25 kg bag per 3–4 m² of board; screw-fixing to timber needs none.
- Joint filler / finishing compound — Roughly 1 kg per 4 m² of joints — check the tub instructions.
Delivery access and what to do with the surplus are the two things that cause a second trip. Neither has a quantity here, because neither is a material.
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). The plasterboard guide works a whole room through, screws and scrim tape included.
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.
Regs & planning for this calculator
These are the rules that decide what goes on the wall, beyond how many sheets you need.
- Approved Document B — fire Walls and ceilings that must resist fire have a required rating, and their surfaces have a lining class. Read the requirement.
- Approved Document E — sound Internal walls and floors within a dwelling have a 40 dB Rw minimum; separating walls between dwellings have an airborne target. Read the requirement.
- Approved Document C — damp Boarding over a wall that is damp is a damp problem, not a boarding problem: the damp-proof course must sit at least 150 mm above adjoining ground. Read the requirement.
- Approved Document L — energy Insulated board and drylining are a common way of meeting the U-value limits for a new or replacement element. Read the requirement.