Gypsum plaster
One-coat plaster bags (e.g. Thistle Universal OneCoat)
- 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 works out bags of one-coat gypsum plaster, which is laid as the undercoat and finished in the same product, for a wall or ceiling area.
Cr is the published coverage per bag at the reference thickness tr (about 2.25 m² at 13 mm for Universal OneCoat), and t is the thickness you will actually lay. A is the area, w the waste, M the bag weight and Wb the water per bag.
Coverage drops in proportion as thickness rises, so the coverage per bag at your thickness is Cr × tr ÷ t. Area divided by that, plus waste, gives the bags, rounded up because bags come whole.
Sense check: an 18 m² wall at 13 mm needs eight bags before waste. The same area as a 2 mm skim would be under two bags, which shows why one-coat work costs far more in material.
Watch out: ceilings are normally laid thinner (about 10 mm), which raises the coverage per bag. Enter the real thickness for each surface.
Bags of a one-coat gypsum plaster (undercoat and finish in one product) for walls or ceilings.
Enter your values to see the result.
Good to know
British Gypsum list Thistle Universal OneCoat at about 2.25 m² per 25 kg bag at 13 mm on walls, 15 L water per bag; ceilings are normally 10 mm, giving about 2.9 m² per bag. Other one-coat products differ: use the figures printed on the bag.
Common mistake
Treating a one-coat plaster like a finish and pricing it at skim coverage: it is laid an order of magnitude thicker, so you need about four times as many bags per m² as a skim.
Limits
Assumes uniform thickness; coverage depends on background flatness and suction.
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.