Gypsum plaster
Skim mixing batch and water-to-plaster ratio
- where
- coverage per bag at your thickness (m²)
- datasheet coverage per bag at the reference thickness (m²)
- reference thickness (mm)
- skim thickness (mm)
- fraction of a bag for this mix
- area you will skim from this mix (m²)
- plaster powder for the mix (kg)
- bag weight (kg)
- clean water for the mix (litres)
- water per full bag from the datasheet (litres)
Explain this equation
This sizes one bucket of skim for the area you can lay and trowel before it sets, and tells you how much water to put in the bucket first. Use it for small jobs, patches or when working alone.
Cb is the coverage of one bag at your thickness, from the datasheet coverage Cr at thickness tr. Bf is the part of a bag the area needs, m is the powder weight and Wt the water. Wb ÷ M is the water-to-plaster ratio, about 0.46 litres per kg for MultiFinish.
The powder needed is a simple proportion of a bag. The water is then the same proportion of the datasheet water per bag, so the mix has the consistency the manufacturer intended whatever the batch size.
Sense check: half a bag of MultiFinish (12.5 kg) skims about 5 m² at 2 mm and takes about 5.75 litres of water, a bit over half a standard bucket.
Watch out: put the water in first and add the plaster to it. Do not add water to a mix that has started to set.
Sizes a single bucket mix for the area you can lay and trowel up before it sets, and gives the water to put in the bucket first.
Enter your values to see the result.
Good to know
British Gypsum: put about 11.5 L of clean water per 25 kg of Thistle MultiFinish in the bucket, then add the plaster (0.46 L per kg). Thistle BoardFinish is also 11.5 L; SprayFinish 12 L. Mix only what you can lay in the setting time (around 1.5 hours for MultiFinish, shortened by high suction and warm weather). Use clean water and a clean bucket every mix: old set plaster speeds the set.
Common mistake
Adding water to plaster rather than plaster to water, or re-tempering a mix that has begun to set; both weaken the finish.
Limits
Datasheet water is a starting point; adjust slightly for workability. Very small mixes are harder to proportion accurately.
Sources
- British Gypsum, Thistle MultiFinish installation instructions (approx. 11.5 L water per 25 kg, 2 mm minimum total)
- 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)
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.