Materials and coverage
Bag weight to yield volume and coverage (V = m / ρ)
- where
- yield volume per bag (litres)
- bag weight (kg)
- yield density, powder per m³ of applied plaster (kg/m³)
- coverage per bag (m²)
- thickness to apply (mm)
Explain this equation
Turns bag weight into the volume of applied plaster it makes and the area it covers at your thickness. Use it to compare products or to work out coverage at a non-standard thickness.
m is the bag weight in kg and ρ the yield density: kilograms of powder per cubic metre of applied plaster. The 1000 converts cubic metres to litres. t is the thickness, and V divided by t gives the area because 1 m² at 1 mm is 1 litre.
Volume is mass divided by density. The yield density is derived from manufacturer coverage, so it already includes the mixing water and any lightweight aggregate.
Sense check: a 25 kg bag of Thistle finish at 1250 kg/m³ makes 20 litres, covering 10 m² at 2 mm, matching British Gypsum's figure. Lightweight HardWall works out at about 758 kg/m³, or 33 litres a bag.
Watch out: this is not the density of the hardened plaster or of the dry powder in the bag.
Turns bag weight into the volume of applied plaster it makes, and the area it covers at your thickness.
Enter your values to see the result.
Good to know
The yield density is the bag weight divided by the volume of plaster it produces once mixed and applied. It can be worked out from any manufacturer coverage figure: British Gypsum finishes (25 kg, 10 m² at 2 mm = 20 L) give 1250 kg/m³; Thistle HardWall (3.0 m² at 11 mm = 33 L) gives about 758 kg/m³ because it contains lightweight aggregate. It is not the same as the set dry density of the hardened plaster.
Common mistake
Using a published dry density of hardened plaster or the bulk density of the powder. Neither tells you how much mixed plaster a bag makes.
Limits
Assumes the yield density implied by the manufacturer's coverage. Water content and mixing affect actual yield slightly.
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.