Work out bags of plaster
Walls (total length × height, less openings) plus any ceiling. Pick the coats and adjust the thickness if your background needs it.
Walls
Openings to take off
Ceiling (0 for none)
Coverage is scaled from the manufacturer's figure at its standard thickness. Bags are 25 kg.
Coverage is approximate and depends on the background and how true it is. Check the manufacturer's current data sheet.
Plaster coverage per bag
Typical coverage for British Gypsum Thistle plasters, the figures the calculator uses.
| Plaster | Thickness | Per bag | kg/m² | Bags / 10 m² |
|---|---|---|---|---|
| Thistle Multi-Finish (skim) | 2 mm | 10 m² | 2.50 | 1 |
| Thistle Hardwall (undercoat) | 11 mm | 3 m² | 8.33 | 4 |
| Thistle Bonding Coat (undercoat) | 8 mm | 3.5 m² | 7.14 | 3 |
Multi-Finish from British Gypsum's merchant data (10 m² at 2 mm). Hardwall about 3 m² at 11 mm; Bonding Coat about 3.5 m² at 8 mm (2.75 m² at 11 mm). Bags per 10 m² is before waste.
How much plaster do I need?
Work out the area in m², then divide by the coverage of one bag at the thickness you are applying. Thicker coats use proportionally more: Hardwall at 11 mm covers about 3 m² a bag, at 13 mm only about 2.5 m².
Skim coat
Thistle Multi-Finish is a finish plaster for both plasterboard and undercoats, laid about 2 mm thick in two passes. A 25 kg bag covers roughly 10 m² at 2 mm. Example: a 4 × 3 m room with 2.4 m walls, a window and a door is about 30.2 m² of wall + 12 m² of ceiling = 42.2 m² → 4.2 bags, 5 with 10% waste.
Undercoats
- Thistle Hardwall — a tough undercoat for most masonry backgrounds, typically 11 mm on walls. About 3 m² per 25 kg bag at 11 mm.
- Thistle Bonding Coat — for low-suction backgrounds such as concrete or painted surfaces treated with a bonding agent, 8 mm on ceilings, up to 11 mm on walls. About 3.5 m² per bag at 8 mm, 2.75 m² at 11 mm.
Uneven backgrounds, chases and dubbing out all use extra — allow more waste on old walls.
Questions plasterers ask
How many m² does a bag of Multi-Finish cover?
A 25 kg bag of Thistle Multi-Finish covers about 10 m² at the standard 2 mm thickness, according to British Gypsum's merchant data. Allow 10% extra for waste.
How much does a bag of Hardwall cover?
About 3 m² per 25 kg bag at the typical 11 mm thickness. Thicker coats cover proportionally less.
How much Bonding Coat do I need?
A 25 kg bag of Thistle Bonding Coat covers about 3.5 m² at 8 mm or 2.75 m² at 11 mm. Divide your area by that figure and add waste.
How many bags of plaster to skim a room?
Add up the wall and ceiling area and divide by 10 for Multi-Finish at 2 mm. A typical 4 × 3 m bedroom with ceiling is around 40 m², so 4 to 5 bags with waste.
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Signed day sheets, variations recorded on site, payment applications and retention tracking for drylining subcontractors.
See Graftum for drylinersCoverage is approximate and depends on the background and how true it is. Check the manufacturer's current data sheet.