Plaster Calculator - How Many Bags of Plaster Do You Need? (UK)
Put in the area you're plastering and whether you're skimming or putting on a bonding undercoat, and this works out how many 25kg bags you need - with wastage allowed for.
Interested? Then → Quotato for Plasterers · More advice on the Blog & Chips guides.
How to use this calculator
Measure the area you're plastering in square metres. Choose skim (multi-finish) for a smooth finish over boards or sound walls, or bonding for an undercoat on a more uneven background. You'll get the number of 25kg bags to buy.
How we calculate this
- We divide your area by the coverage from one bag, round up to whole 25kg bags, and add a wastage margin.
- Multi-finish covers ~10 m² per 25kg bag at a 2mm two-coat skim; bonding covers ~5 m² per bag at the thicker 8-11mm undercoat.
- We add 10% for wastage - what sticks in the bucket, goes off, or gets binned when it tightens up.
Worked example
Skimming a 12 m² wall with multi-finish: 12 ÷ 10 = 1.2 bags, so 2 bags. If it needed a bonding undercoat first, 12 ÷ 5 = 2.4 bags, rounding up to 3 bags - bonding goes on thicker so it eats bags faster.
UK notes & gotchas
- Coverage figures suit common UK gypsum plasters (e.g. British Gypsum Thistle) at typical thicknesses.
- Multi-finish is your finishing skim at ~2mm in two coats; bonding is an undercoat for uneven or low-suction backgrounds.
- Plaster has a working life of ~30-90 minutes, so don't gauge up more than you can lay on in time.
FAQ
Does this cover both undercoat and skim?
It works out one coat type at a time. If you're bonding then skimming, run it twice and order both sets of bags.
How accurate is the coverage?
We use ≈10 m²/bag for multi-finish and ≈5 m² for bonding, matching typical UK bag figures. Real coverage shifts with thickness and background, so the 10% wastage gives headroom.
Why 25kg bags?
That's the standard sack size for UK gypsum plasters, so the calculator works in whole 25kg bags.
Does wastage really need to be 10%?
It's a sensible default - plaster left in the bucket or going off early eats into your bags.
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