How this is worked out
A 25kg bag of British Gypsum Thistle Multi-Finish covers about 10 m² at 2mm. We scale that by your thickness (a 3mm coat covers two thirds as much) and add the waste allowance.
Other finish plasters are close, but check the bag. Backing plasters such as Hardwall cover far less per bag because they go on 8 to 11mm thick, so this calculator isn’t right for those.
Worked example
A bedroom 3.5m × 3.2m with 2.4m walls: the walls are (3.5 + 3.2) × 2 × 2.4 = 32.2 m². Take off a door (1.6 m²) and a window (1.4 m²) and it’s 29.2 m². At 10 m² per bag plus 10%, that’s four bags.
Measuring tips
Leave out doors and big windows, but don’t take off small items like sockets. Ceilings are length × width. If you’re new to plastering, the waste allowance matters: first attempts often mix too much and lose the end of the bucket.
Questions people ask
How many bags of plaster do I need to skim a room?
A 25kg bag of Thistle Multi-Finish covers about 10 m² at 2mm. Four walls of a 3.5m × 3.5m room with a 2.4m ceiling height are about 33 m² before doors and windows, so four bags with a little to spare.
How thick is a skim coat?
About 2mm, usually in two thin coats. British Gypsum bases its Multi-Finish coverage on 2mm.
Can I skim straight onto old paint?
Usually not without preparation. Glossy or flaky paint needs scraping and a bonding primer first, or the plaster can fail to grip.
How long does a bag of plaster last once opened?
Plaster goes off in damp storage and has a use-by date on the bag. Old plaster sets too fast and can blow, so buy what you need close to the job.
Sources
Trade review pending. A named tradesperson will check this calculator, and their name and qualifications will appear here.
Figures last checked 25 October 2026. Read how we calculate and our disclaimer. Found a mistake? Tell us.