Gypsum plaster
Backing / undercoat plaster bags (Thistle BondingCoat, HardWall, DriCoat, ToughCoat)
- where
- coverage per bag at your thickness (m²)
- datasheet coverage per bag at the reference thickness (m²)
- reference thickness for that coverage (mm)
- your average thickness (mm)
- bags needed, unrounded
- area to plaster (m²)
- waste allowance (%)
- whole bags to buy
- plaster needed (kg)
- bag weight (kg)
- mixing water needed (litres)
- water per bag from the datasheet (litres)
Explain this equation
This gives the number of bags of gypsum undercoat, such as BondingCoat or HardWall, for an area at a chosen thickness, plus the plaster weight and water. Use it when pricing a float and set job or patch repairs.
Cr and tr are the coverage and thickness printed by the maker (for example 2.75 m² per bag at 11 mm for BondingCoat). t is the thickness you expect to lay, A the area and w the waste. Wb is the water per bag, which differs between products.
Each bag fills a fixed volume, so the coverage at your thickness is Cr × tr ÷ t. The area divided by that coverage gives the bag count, increased by the waste percentage and rounded up to whole bags.
Sense check: a 3 m × 2.4 m wall (7.2 m²) at 11 mm in BondingCoat needs about 2.6 bags, so three. On a ceiling at 8 mm the same bag goes about 40% further.
Watch out: undercoat thickness on old walls is rarely uniform. Check depths with a straight-edge in several places and use the average, not the thinnest spot.
Number of 25 kg bags of gypsum undercoat for a given area and thickness, from the manufacturer’s coverage at its stated thickness.
Enter your values to see the result.
Good to know
British Gypsum selector guide figures per 25 kg bag at 11 mm (walls): BondingCoat about 2.75 m² (14 L water), HardWall about 3.0 m² (15 L), DriCoat about 3.25 m² (13 L). Ceilings are normally 8 mm, which raises coverage by 11/8. For ToughCoat or other brands, enter the coverage and water printed on the bag. Allow a finish coat on top (about 2 mm) in your total build-up.
Common mistake
Pricing an undercoat at 11 mm when the wall is out of plumb and averages 15–20 mm in places. Take several depth readings with a straight-edge before ordering.
Limits
Assumes uniform thickness. Coverage figures are approximate and depend on background suction and flatness. Choose the product for the background (e.g. BondingCoat for low-suction surfaces, HardWall for high suction) using the manufacturer’s selector.
Sources
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.