Skim plaster needed
4 bags
30.2 m² · 79.7 kg, in 25 kg bags
Your figures, your sums - nothing is sent anywhere. Skim coat (finish plaster, Multi-Finish type), usually 2–3 mm on top of the base coat: about 1.2 kg per m² for each mm (a bag covers about 10 m² at 2 mm); check your bag for its own coverage. Bonding, beading and scrim are extra. Doors count as 1.9 m², windows as 1.5 m².
How much plaster a room needs
Work out the area to skim: the distance round the room (twice the length plus twice the width) multiplied by the height, less about 1.9 m² for each door and 1.5 m² for each window. Add the ceiling (length × width) if you are skimming it too.
Finish plaster such as Multi-Finish goes on about 2 mm thick, and uses roughly 1.2 kg per m² for every millimetre (a 25 kg bag covers about 10 m² at 2 mm). Divide the total by the bag size (25 kg) and round up.
A worked example
A room 4 m × 3 m with a 2.4 m ceiling height has 33.6 m² of wall. With one door and one window taken off, that is 30.2 m².
At 2 mm that is about 72 kg, and 10% for mixing losses makes about 80 kg. That is four 25 kg bags.
What changes the amount
Uneven or very absorbent walls take more. Old, dusty or painted surfaces need a bonding coat first, which is a different product.
Different brands have their own coverage figure on the bag. Use it if it differs from the typical figure here.
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Questions
How many bags of multi finish for a room?
A 4 × 3 m room with 2.4 m walls, one door and one window, skimmed at 2 mm, takes about four 25 kg bags. Add the ceiling and it is about five.
How thick should a skim coat be?
Finish plaster is normally applied 2 to 3 mm thick in two thin coats. Thicker coats crack.
How far does a 25 kg bag go?
About 10 m² at 2 mm, but the bag states its own figure. Rough or absorbent walls cover less.