How many plasterboard sheets do I need?

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.

Worked example: a 4 m × 3 m room

A 4 m × 3 m room with 2.4 m walls, four walls and the ceiling, in 2400 × 1200 mm boards, with 10% waste.

How we got there

1. Wall run 14 m × 2.4 m = 33.6 m² — The four walls measure 14 m around and 2.4 m high, giving 33.6 m².
2. Plus the ceiling: 12 m² — The 4 m × 3 m ceiling adds 12 m², for 45.6 m² in total.
3. +10% waste = 50.16 m² — The waste allowance covers offcuts at openings and the last board in each run.
4. 18 boards — A 2400 × 1200 mm board covers 2.88 m², so 50.16 m² needs 18 sheets.
5. Screws and tape — 1,368 screws is 2 boxes of 1,000; the joints need 69 m of tape, so 1 roll.

What you need to buy

The order for the worked example above, straight from this calculator's own shopping list.

• Plasterboard 2,400 × 1,200 mm, 12.5 mm — 18 sheets (One extra sheet is cheaper than a second trip.)
• Drywall screws 32 mm (box of 1,000) — 2 boxes (≈ 1,368 screws at 30 per m².)
• Scrim (joint) tape, 90 m roll — 1 roll (≈ 69 m for joints and corners.)
• Corner bead (external corners) — as needed (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) — as needed (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 — estimate only (Roughly 1 kg per 4 m² of joints — check the tub instructions.)

Figures for the worked example only. Change the measurements above and the list recalculates.

Waste and over-ordering

The 10% waste allowance covers offcuts at openings and the part-board at the end of each run. A room with many windows and doors wastes more; a plain wall wastes less.

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 R_w 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.