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Free plasterer calculator
Plaster calculator: bags of MultiFinish, HardWall, bonding, one-coat and render
Measure the walls and ceiling, take off the doors and windows, pick the system and see how many bags to order. Coverage comes from the British Gypsum data sheets: 10m² a bag for Thistle MultiFinish and BoardFinish at 2mm, 3m² for HardWall and 2.75m² for BondingCoat at 11mm, 2.25m² for Universal OneCoat at 13mm, all with 10% wastage. Render is worked out as sand and cement by weight, a bagged base coat, or monocouche.
What you need
Plaster
- Thistle HardWall (walls)
- 12 × 25kg bagsThistle HardWall, 11 mm undercoat on brick and block: 3m² a bag over 30.1m² plus 10%British Gypsum product data sheet
- Thistle BondingCoat (ceiling)
- 4 × 25kg bagsThistle BondingCoat, 8 mm on ceilings: 3.5m² a bag over 12.0m² plus 10%British Gypsum product data sheet
- Thistle MultiFinish skim
- 5 × 25kg bagsThistle MultiFinish, 2 mm skim over undercoat: 10m² a bag over 42.1m² plus 10%British Gypsum product data sheet
Sundries
- PVA (diluted)
- 9 litresabout 5m² a litre; British Gypsum specify Bond-it on low suction and GypPrime on high suction, PVA is trade habit
- Scrim tape
- 1 × 90m rollsabout 15m of board joints in the ceiling
- Clean water
- 294 litres11.5 litres a bag for finish plasters, 14 to 15 for undercoatsBritish Gypsum product data sheets
- Net area
- 42.1 m²30.1m² of wall and 12.0m² of ceiling, after 3.5m² of openings
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
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Or claim your free profile link nowHow the maths works
- Area is total wall length × height, less the doors and windows, plus the ceiling for the plaster systems. 10% wastage goes on before bags are worked out, and bags round up.
- Coverage per 25kg bag is from the British Gypsum product data sheets: Thistle MultiFinish and BoardFinish 10m² at 2mm; HardWall 3m² at 11mm; BondingCoat 2.75m² at 11mm on walls and 3.5m² at 8mm on ceilings (8mm is also the maximum on plasterboard); Universal OneCoat 2.25m² at 13mm.
- Float and set (two-coat) on bare brick and block is HardWall then a MultiFinish skim. BondingCoat is the undercoat for concrete, plasterboard and other low-suction backgrounds. Plasterboard only needs BoardFinish. One-coat is for patching and small areas.
- Sand and cement render is not sold in bags. Two coats, a 10mm scratch coat and a 7mm top coat, take about 27kg of sand per m² and roughly 9 bags of cement per tonne of sand at 1:4 scratch and 1:5 top (tradecalculator), so the sand comes as bulk bags of about 800kg. A bagged base coat such as K-Rend HP12 goes 1.8kg per mm per m², about 2.3m² a bag at 7mm (K-Rend TDS). Monocouche is about 1m² a bag at 15mm.
- Primer is about 5m² a litre diluted. British Gypsum specify Thistle Bond-it on low-suction backgrounds and GypPrime on high suction; PVA is what most of the trade uses. Dot and dab uses about 3m² of DriWall adhesive a bag.
- Angle beads run up every external corner: corners × wall height in 2.4m beads. Scrim covers the board joints in a ceiling, about 3.6m per 2.88m² board, in 90m rolls.
- Water is 11.5 litres a bag for the finish plasters, 14 for BondingCoat and 15 for HardWall and OneCoat. Plaster has a 4-month shelf life, so buy it fresh.
Worked example
A 4m × 3m room with 2.4m walls has 33.6m² of wall and 12m² of ceiling. Take off 3.5m² for a door and window and that is 42.1m² net. Float and set needs 12 bags of HardWall for the walls, 4 bags of BondingCoat for the ceiling and 5 bags of MultiFinish over the lot, plus 9 litres of PVA, 1 roll of scrim and about 294 litres of water.
Questions people ask
How many square metres does a bag of multi-finish cover?
10m² at a 2mm skim, which is the British Gypsum figure for Thistle MultiFinish and BoardFinish. The calculator adds 10% for wastage on top. Over rough bonding or an uneven wall you use more, so it rarely hurts to order one extra.
HardWall or bonding under a skim?
On plasterboard, neither: skim straight onto the board with BoardFinish. On bare brick and block use HardWall, the normal undercoat for masonry, at 3m² a bag. BondingCoat is for low-suction backgrounds like concrete, painted surfaces and plasterboard that needs building out, at 2.75m² a bag, or 3.5m² at 8mm on ceilings. Browning is discontinued.
What is the difference between one-coat plaster and a two-coat system?
One-coat is a single product that goes on thick and gets a finish in the same pass, handy for patching and small areas, but it only covers 2.25m² a bag at 13mm so it is dear on a whole room. Two-coat (float and set) is the normal way: an undercoat to level, then a thin finish for the smooth surface.
How much render do I need?
Sand and cement render is two coats: a 10mm scratch coat at about 16kg of sand per m² and a 7mm top coat at 11kg, so 27kg of sand per m² and about 9 bags of cement per tonne of sand. A bagged base coat like K-Rend HP12 does about 2.3m² a bag at 7mm, and monocouche about 1m² a bag at 15mm. Pick the system and the calculator does the right one; you do not run it twice.
Do I need to PVA the wall first?
On high-suction backgrounds like old plaster or aircrete, a diluted coat controls suction so the skim does not dry before you can trowel it. British Gypsum would rather you used Thistle Bond-it on low-suction surfaces and GypPrime on high suction, but diluted PVA is what most plasterers use at about 5m² a litre.
How much does it cost to plaster a room?
Materials are cheap next to labour: this calculator gives the bags, but the price of the job depends on the state of the walls and where you are. See the cost guide for a room and get two or three quotes.
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