Materials and coverage
Bags of plaster for an area at any thickness (from published coverage)
- where
- bags needed before rounding up
- area to plaster (m²)
- average thickness applied (mm)
- published coverage per bag (m²)
- thickness the coverage is quoted at (mm)
- waste allowance (%)
Explain this equation
Converts a manufacturer's coverage figure to the thickness you will actually apply, and works out how many bags to order. Use it when ordering backing plaster or finish for a job.
A is the area and t the average thickness you will apply. A0 is the published coverage per bag and t0 the thickness it is quoted at. w is the waste allowance in percent. n is the bag count before rounding up.
Each bag makes a fixed volume of plaster, A0 × t0. The job needs a volume of A × t, so the ratio of the two gives the number of bags; the bracket adds the waste allowance.
Sense check: HardWall covers about 3 m² per bag at 11 mm, so 20 m² at 11 mm with 10% waste needs about 7.3 bags, order 8. Applying 3 mm finish instead of 2 mm cuts coverage from 10 m² to about 6.7 m² a bag.
Watch out: walls out of plumb push the average thickness up. Measure before ordering.
Converts a manufacturer's coverage figure (m² per bag at a stated thickness) to the thickness you will actually apply and the bags to order.
Enter your values to see the result.
Good to know
British Gypsum White Book coverage per 25 kg bag: Thistle MultiFinish, BoardFinish and similar finishes about 10 m² at 2 mm; HardWall about 3.0 m², BondingCoat about 2.75 m² and DriCoat about 3.25 m² at 11 mm; Universal OneCoat about 2.25 m² at 13 mm. Coverage scales inversely with thickness, because a bag yields a fixed volume. Uneven backgrounds and dubbing out push the average thickness, and so the bag count, up.
Common mistake
Using the 2 mm finish coverage for a 3 mm skim over a poor surface, or the 11 mm backing coverage where walls are out of plumb by 10 mm or more. Measure the real average thickness.
Limits
Assumes the bag yields the same volume at any thickness. Ignores mixing waste beyond the allowance and site losses. Coverage figures are manufacturer approximations.
Sources
- British Gypsum White Book: Thistle plaster finishes (coat thicknesses, coverage per 25 kg bag, dry set weights, minimum temperature 2 °C until dry, plaster must be thoroughly dry before decorating)
- British Gypsum product data sheet: Thistle DriCoat (3.25 m² per 25 kg bag at 11 mm; 24 h between coats)
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.