Materials and coverage
Average thickness achieved from bags used (thickness = volume / area)
- where
- volume of applied plaster (litres; 1 m² × 1 mm = 1 L)
- bags used
- published coverage per bag (m²)
- thickness the coverage is quoted at (mm)
- average applied thickness (mm)
- area covered (m²)
Explain this equation
Back-calculates the average thickness actually applied from the bags used and the area covered. Use it to check work against the specification or to support a pricing variation.
nb is the number of bags used, A0 the published coverage per bag and t0 the thickness it is quoted at, so nb × A0 × t0 is the total volume V. A is the area covered and t the average thickness.
Thickness is volume divided by area. Conveniently, 1 m² at 1 mm thick is exactly 1 litre, so m² × mm gives litres directly.
Sense check: five bags of finish (10 m² at 2 mm each, 20 litres per bag) over 40 m² is an average of 2.5 mm. Twelve bags of HardWall over 30 m² is 13.2 mm.
Watch out: do not count plaster that was mixed and wasted, or the thickness will be overstated.
Back-calculates the average thickness actually applied from the number of bags used and the area covered, to check against specification or price.
Enter your values to see the result.
Good to know
One m² at 1 mm is exactly 1 litre, so coverage per bag × quoted thickness gives the bag's yield in litres. Using British Gypsum figures, a 25 kg bag of finish yields about 20 litres and a bag of HardWall about 33 litres. If the average comes out well above the specified 2 mm skim or 11 mm backing, the background was poorer than allowed for, which is worth recording for pricing variations.
Common mistake
Including mixed-but-wasted plaster in the bag count, which overstates thickness.
Limits
Average only: does not show local high or low spots. Accuracy depends on the manufacturer's coverage figure.
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.