Plaster & Board Calculator
Calculate plasterboard sheets, plaster bags and fixings for any room. Covers full board and skim, skim only and board only — with dot-and-dab or stud fixing. 2026 UK pricing.
Dot-and-dab: plasterboard adhesive applied in dabs directly to masonry. ~4–5 kg adhesive per board. Boards cannot be weight-bearing.
Multi-finish: standard skim plaster applied 2–3mm over pre-sealed plasterboard. ~12 m²/bag.
0.9×2.0m each
1.2×1.0m each
Plastering Guide
How to Calculate Plasterboard & Plaster
How to calculate plasterboard sheets
Divide your net wall or ceiling area by the sheet coverage. A standard 2400×1200mm sheet covers 2.88 m². Always round up to whole sheets — you cannot buy fractions.
Worked example: A 12 m² bedroom wall ÷ 2.88 m² per sheet = 4.16 sheets → round up to 5 sheets. At 10% waste: 12 × 1.10 ÷ 2.88 = 4.58 → order 5 sheets. For a full room (walls + ceiling), use the calculator above — it handles all four walls, ceiling, and opening deductions automatically.
How much skim plaster do I need?
Multi-finish and board finish skim plasters cover approximately 10 m² per 25kg bag at 2mm. This is a planning figure, not a guarantee — coverage varies with application thickness and substrate absorbency.
Divide your net area by 10 and round up, then add one spare bag. A 40 m² room needs 40 ÷ 10 = 4 bags + 1 spare = 5 bags. If applying two coats (not usually needed over board), double the quantity. Coverage figures comply with BS EN 13279-1 (gypsum binders and gypsum-based finishing plasters).
Choosing the right plasterboard type
Standard (grey/ivory): General internal walls and ceilings. 9.5mm for ceilings and over-boarding; 12.5mm for walls; 15mm for acoustic and fire resistance upgrade.
Fire-rated (pink): Required by Building Regulations Part B in garages adjacent to the house, around boiler enclosures, and in certain commercial applications. Typically 12.5mm or 15mm.
Moisture-resistant (green): For areas behind tiling in bathrooms and kitchens. It is moisture-tolerant, not waterproof — tile and grout are still required to seal the surface.
Acoustic (soundbloc / blue): Required for party walls and separating floors under Building Regulations Part E. Higher density core provides better Rw values. Check stud spacing specification — acoustic boards are heavier and need closer centres.
Boarding and skimming vs wet plaster — which is better?
Board and skim is the UK standard for most domestic new build and renovation. It is faster than wet plaster, shrinks less as it dries, and is cheaper per m² for larger areas. A plasterer can skim a full room in one day.
Wet plaster (sand and cement render, hardwall, or browning undercoat + skim) is used over masonry, brick, block, and concrete where plasterboard would not bond. It costs more in materials and labour time but gives a harder, more durable finish. Use the Mortar Calculator to estimate backing coat materials for blockwork walls before skimming.
High-suction backgrounds and base coats
Bare brick, blockwork, and concrete are too porous for a direct skim. A bonding coat (8–10mm) or hardwall undercoat must be applied first, allowed to dry to a firm surface, then finish skim applied on top. Hardwall is preferred for high-suction backgrounds — it is more resistant to suction pull than standard bonding coat.
Fresh grey plasterboard must be sealed with diluted PVA (5:1 water to PVA) or a board sealer before skimming. Never skim onto dry, unsealed board — the board absorbs moisture from the skim before it bonds, causing the finish to crack and fall. Apply PVA, allow it to go tacky, then skim immediately. This process is described in BS 8212 (Code of practice for drylining and partitioning).
Wastage and rounding — why you always need more
Plasterboard waste comes from cuts around door frames, window reveals, electrical sockets, and the unavoidable off-cuts when sheets don't fit perfectly. Professionals add 10% wastage for straightforward rooms and 15% for complex rooms with sloped ceilings or many openings.
For plaster, always buy one bag more than the calculation suggests. A bag opened and partially used cannot be resealed effectively — moisture from the air begins hydrating the gypsum and will shorten the remaining shelf life. Budget plaster is usually made to BS EN 13279-1 Class B2 and has a shelf life of 6 months from manufacture when stored dry.
Ceilings vs walls — what's different
Use 9.5mm board for ceilings — it is lighter and reduces the load on ceiling joists. Fix at 230mm centres with 38mm drywall screws, staggered across joist lines. Never exceed 600mm joist centres with 9.5mm board.
Over-boarding (applying new board over existing plaster ceilings) is common in renovations. Use 9.5mm over sound existing plaster, fixing back into joists with longer screws (65mm). Check the existing ceiling for signs of moisture damage or loose keys before over-boarding — a failing ceiling behind new board is a safety risk. Over-boarding adds approximately 10kg/m² to the ceiling load.
FAQ
Common Questions
A standard 2400×1200mm sheet covers 2.88 m². For a 12 m² wall: 12 ÷ 2.88 = 4.16, round up to 5 sheets. With 10% wastage allowance you need 12 × 1.10 ÷ 2.88 = 4.58 → 5 sheets. For rooms with lots of cuts (chimney breasts, windows) add 15%.
A typical 3m × 3m bedroom has roughly 26 m² of net wall area. At 10 m² per 25kg bag of multi-finish skim, you need 3 bags — order 4 to be safe. Always have one spare bag: opened bags absorb moisture and have a short shelf life once unsealed.
Use 9.5mm for ceilings — it is lighter (reducing joist load) and easier to handle overhead. Fix at 230mm centres. Use 12.5mm for walls; it is the standard domestic thickness and provides better impact resistance. 15mm is used for acoustic and fire upgrade applications.
Not directly with finish plaster. Bare brick is too porous and uneven. Apply a bonding coat or hardwall undercoat (8–11mm) first, allow to set firm, then apply finish skim (2–3mm) on top. Hardwall is the preferred undercoat on high-suction backgrounds like engineering brick or dense blockwork.
Standard kitchens do not require fire-rated board. Fire-rated (pink) plasterboard is required by Building Regulations Part B in integral garages that share a wall or ceiling with the living space, and around boiler enclosures and escape routes in some configurations. Always check your local authority's requirements if in doubt.
Fresh skim plaster turns from dark pink to a uniform pale buff when dry — typically 2–4 weeks depending on room temperature and ventilation. Do not rush: painting over damp plaster traps moisture, causes efflorescence, and leads to peeling. Apply a mist coat (diluted 70:30 emulsion to water) as the first coat to seal the surface without blocking it.
Board finish (e.g. Thistle Board Finish) is formulated specifically for plasterboard substrates — it has a slightly slower set and is better suited to experienced plasterers. Multi-finish is more versatile, working over both board and sanded backing coats. Both cover ~10 m² per 25kg bag at 2mm. Multi-finish is the most commonly specified for domestic projects.
Building Regulations Part E requires party walls between dwellings to achieve a minimum airborne sound insulation of 45dB Rw (new build) or 43dB Rw (material change of use). Standard 12.5mm board alone will not meet this. Soundbloc (or equivalent 15mm high-density board) combined with acoustic mineral wool in the cavity and resilient bars is the typical compliant solution. Always commission a pre-completion acoustic test if building new.
Next Steps
Tools for the rest of the job
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