Plaster calculator
Estimate plaster bags, sand & cement render quantity or plasterboard sheets for walls and ceilings. Pick your finish, enter the area and we'll return bag counts and wastage figures ready for a trade quote.
Quantity needed
3 bags
Includes 10% wastage allowance.
Tip: coverage is approximate. Check the product specification for the exact coverage of your chosen plaster, render or board.
Plaster, render or plasterboard?
Use the calculator to estimate three common finishes. For skim-only work on plasterboard, choose finish plaster— one 25 kg bag covers roughly 10–11 m² at 2 mm. For two-coat work on blockwork, choose backing plasterat 11 mm followed by a 2 mm skim. For external walls, switch to render and enter the total depth across scratch and float coats.
Plasterboard sheet counts
Standard 2.4 × 1.2 m wallboard covers 2.88 m² per sheet. For ceilings use 2.4 × 1.2 m plasterboard at 2.88 m² or 1.8 × 0.9 m for awkward sizes. Add 10% for off-cuts on rectangular rooms and 15% on irregular layouts with dormers or stair wells.
Trade tips
- Always check suction on the background — high-suction blocks need a water spray or bonding agent.
- Allow 24 hours between backing and skim coats in normal drying conditions.
- For external render, allow 1 mm extra per coat for wastage on hawk and board losses.
- Buy plasterboard in full packs where possible — trade pack pricing is significantly cheaper than break-pack.
Common projects
Quick reference quantities for the jobs we get asked about most. Enter your own measurements in the calculator above for an exact figure.
Skim two walls
Two walls each 3 m × 2.4 m give roughly 15 m² and need about 2 bags of finish plaster.
Plasterboard ceiling
A 4 m × 5 m ceiling needs roughly 7 sheets of 2.4 m × 1.2 m plasterboard.
Render 10 m² of blockwork
Two-coat render on 10 m² of blockwork needs roughly 5–6 bags of render.
Patch repair
A 2 m² patch repair usually needs about one 25 kg bag of finish plaster.
Frequently asked questions
- How many square metres does a 25 kg bag of plaster cover?
- A 25 kg bag of finish plaster covers roughly 10 m² at a 2–3 mm skim thickness.
- What is the difference between plaster and render?
- Plaster is for internal walls and ceilings. Render is applied externally to protect walls from the weather.
- Do I need PVA or a bonding agent?
- High-suction backgrounds such as bare blockwork usually need a coat of PVA or a bonding agent to control suction and improve adhesion.
- How thick should a skim coat be?
- A skim coat over plasterboard is normally 2–3 mm. Two-coat work on blockwork is 11 mm backing plus a 2 mm skim.
- Can I plaster over painted walls?
- Gloss or sealed paint must be sanded or treated with a bonding agent. PVA is often used to key the surface before skimming.