Free tool · BuildMeA Projects
Plaster & Render Calculator
Order too few bags and the job stops while the mix goes off; order too many and they're setting solid in the van. Enter your wall area, pick the product, and get an honest bag count with wastage already built in.
With 10% wastage that's 0 m² of material to allow for. Round up — you can't half-order a bag.
Every figure here is an estimate. Coverage varies with the background, the thickness you lay it on and your technique — always round up and keep a bag spare for making good.
How much plaster do I need per m²?
It depends entirely on the product and how thick you're laying it. A 25kg bag of multi-finish skim, applied as two thin coats at around 2mm, covers roughly 10 m² — so about 0.1 of a bag per m². Undercoat plasters go on far thicker: bonding at 8–11mm covers around 5 m² a bag, and hardwall at 11mm around 4.5 m². Divide your area by the coverage on the bag, add a little for wastage, and round up. This calculator does exactly that.
How many bags of plaster to skim a room?
For skim, work out the total wall area — length × height for each wall, added together — then divide by about 10 m² per 25kg bag of multi-finish. A typical 4m × 3m room with 2.4m walls is roughly 33 m² of wall, so around 3.5 bags before wastage, or 4 bags once you round up and allow 10%. Add the ceiling (length × width) if you're skimming that too. Always keep a spare bag: running out mid-skim means a visible join where the first batch has already set.
What's the difference between bonding and skim plaster?
They do different jobs and go on in a specific order. Bonding is an undercoat (or "backing") plaster laid thick — usually 8–11mm — to build out and level a wall, and it grips well on smoother, less absorbent backgrounds like painted surfaces or concrete. Skim (multi-finish) is the thin 2mm top coat that gives you the smooth, paint-ready finish. On most walls you'd apply an undercoat such as bonding or hardwall first, let it firm up, then skim over it. That's why this tool lists them separately, each with its own coverage.
Boarding or building the wall first?
If the wall isn't boarded yet, our Plasterboard Calculator works out the sheets, screws and jointing compound before you skim. Rendering or plastering a new brick or block wall instead of an existing one? Our Brick & Block Calculator works out the bricks, blocks and mortar for the wall itself.
Does this tool store my details?
No. There's no sign-up and nothing to submit — every calculation runs entirely in your browser as you type. Nothing you enter is uploaded, saved or sent anywhere, so you can use it as many times as you like without leaving a trace.
Please note: this tool gives a guide or estimate only. It may not be 100% accurate and is not professional advice — always check the figures before you rely on them. Nothing you enter is uploaded — the calculation runs entirely in your browser.
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