How this calculator works
Worked example: 8 m of wall 2.4 m high is 19.2 m², and with 3 m² of openings that is 16.2 m². At 2 mm one 25 kg bag of finishing plaster covers 10.0 m², so with 10% extra you need 2 bags (50 kg). Bonding coat at 11 mm on the same wall covers 2.75 m² a bag and needs 7 bags. On a ceiling at 8 mm it covers 3.50 m² a bag. A skim of 3 mm instead of 2 mm needs 3 bags. With no openings the finish would be 3 bags.
Bags of finishing plaster (25 kg, 2 mm skim, 10% extra) for common areas
The area of wall or ceiling and the bags to buy.
| Area | Bags of finishing plaster | Bags of bonding coat on a wall (11 mm) |
|---|---|---|
| 10 m² | 2 | 4 |
| 20 m² | 3 | 8 |
| 30 m² | 4 | 12 |
| 40 m² | 5 | 16 |
| 60 m² | 7 | 24 |
| 80 m² | 9 | 32 |
Tips before you buy
- Measure the walls and ceiling separately, and take off the doors and windows that are bigger than about 1 m².
- Use the coverage on the bag you buy, and the thickness it says. The figures here are for the common bags.
- Mix only what you can use in the time on the bag. Plaster that starts to set cannot be saved.
- Prime new plasterboard or a very dry background first, with PVA or a primer the maker recommends.
- A thicker bonding coat is for uneven walls: build it up in layers, not in one 20 mm pass.
Tools & materials for this project
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Frequently asked questions
How much plaster do I need?
Divide the area by the coverage of a bag at your thickness: for 16.2 m² at 2 mm, 2 bags of 25 kg of finishing plaster with 10% extra.
How far does a 25 kg bag of finishing plaster go?
About 10 m² at 2 mm thick. A skim of 3 mm covers about 6.7 m² a bag.
How far does a bag of bonding coat go?
About 2.75 m² a bag on a wall at 11 mm, and 3.50 m² on a ceiling at 8 mm.
How thick should plaster be?
A skim or finishing coat is about 2 mm. A bonding coat is 8 to 11 mm, and a whole wall is often 11 mm of bonding coat with a 2 mm skim over it.
Do I take off the doors and windows?
Yes, take off any opening bigger than about 1 m², but keep the allowance for waste, since the plaster you lose to the floor and the hawk is real.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.