Skim Coat Calculator — How Much Multi-Finish Do I Need?
Calculate how many bags of multi-finish plaster you need for skim coating walls and ceilings. Based on UK 25kg bag sizes.
Enter the total surface area to be skim coated
Adds 10% extra for mixing and application losses
10% for experienced plasterers, 15-20% for DIY
Enter your supplier price for a cost estimate
How We Calculate This
Our skim coat calculator divides your total wall or ceiling area by the coverage rate of multi-finish plaster based on your surface type. Coverage rates are based on British Gypsum Thistle Multi-Finish guidelines at the standard 2mm application thickness.
The formula
Bags needed = (Total area ÷ Coverage per bag) × Wastage factor
For example, skimming 20 m² of plasterboard at 10 m²/bag: 20 ÷ 10 = 2.0 bags → round up to 2 bags, plus 10% wastage margin = 3 bags of 25kg multi-finish.
Coverage rates by surface
- Plasterboard: 10 m² per 25kg bag at 2mm — smooth, consistent surface absorbs less
- Backing coat: 10 m² per 25kg bag at 2mm — standard rate over bonding or hardwall
- Old plaster / patched walls: 8 m² per 25kg bag at 2mm — uneven surfaces require more plaster
These rates assume a 2mm skim coat thickness as recommended by British Gypsum. The 10% wastage factor covers mixing losses, dropped plaster, and filling minor imperfections. If you are new to skimming, consider 15-20% wastage.
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.