Gypsum plaster
Finish plaster bags (skim, e.g. Thistle MultiFinish)
- 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 how many 25 kg bags of finish plaster you need for a skim, and how much water to have ready.
Cr is the coverage the manufacturer prints for one bag at their reference thickness tr (about 10 m² at 2 mm for MultiFinish). Cb is what one bag really covers at your thickness t. A is the area, w the waste allowance, M the bag weight and Wb the water per bag from the datasheet.
A bag holds a fixed volume of plaster, so laid twice as thick it covers half the area: coverage scales with tr ÷ t. Area divided by Cb gives bags, the waste term adds a percentage, and the result is rounded up to whole bags. Weight and water use the unrounded count.
Sense check: a 4 × 3.5 m ceiling (14 m²) at 2 mm is about 1.4 bags, so two bags with waste. Go to 3 mm and the same ceiling needs about 2.1 bags plus waste, so order three.
Watch out: the thickness you enter should be the average over the whole area, not the 2 mm you aim for on a flat board.
Number of 25 kg bags of finish plaster for a skim, with the published coverage scaled to the thickness you actually lay on.
Enter your values to see the result.
Good to know
British Gypsum give Thistle MultiFinish and BoardFinish as about 10 m² per bag at 2 mm, with 11.5 L of water per bag; SprayFinish about 11 m² at 2 mm with 12 L. Coverage falls in direct proportion as thickness rises: 3 mm gives two-thirds of the 2 mm coverage. A 10% waste allowance covers droppings, mixing residue and uneven backgrounds; increase it on poor walls.
Common mistake
Using the 2 mm datasheet coverage for a skim over an uneven wall that really averages 3 mm, so you run out a third of the way short.
Limits
Linear scaling assumes the same set density at any thickness. Datasheet coverage is for a flat, sound background; real coverage depends on background flatness and suction.
Sources
- British Gypsum, Thistle MultiFinish product data sheet (10 m² per bag at 2 mm, 11.5 L water per bag)
- British Gypsum, Thistle Essential Selector Guide (coverage, thickness and water per 25 kg bag)
- British Gypsum, Thistle MultiFinish installation instructions (approx. 11.5 L water per 25 kg, 2 mm minimum total)
- BS EN 13279-1 Gypsum binders and gypsum plasters (product standard for Thistle plasters)
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.