Render and lime
Lime plaster mix (lime putty and sand by volume)
- where
- wet plaster volume (m³)
- area to plaster (m²)
- coat thickness (mm)
- waste allowance (%)
- loose material volume (m³)
- dry-volume factor, loose materials ÷ finished plaster
- lime putty (litres)
- lime putty parts
- sand parts
- sand volume (m³)
- sand mass (tonnes)
- sand bulk density (kg/m³)
Explain this equation
This splits one coat of traditional lime plaster into litres of lime putty and cubic metres or tonnes of sharp sand, for a mix by volume such as 1:2.5 or 1:3.
V is the finished volume of the coat. k allows for the fact that the loose ingredients take up more room than the finished plaster. l and s are the putty and sand parts; the putty share is multiplied by 1000 to give litres. ρs turns the sand volume into tonnes.
In a 1:3 lime mix the putty mostly fills the air gaps between the sand grains, which make up roughly a third of the sand volume. So four buckets of ingredients give about three buckets of plaster, which is the same 4 ÷ 3 ≈ 1.33 factor used for cement mixes.
Sense check: 20 m² at 15 mm is 0.3 m³ of plaster; at 1:3 that is about 100 litres of putty and 0.3 m³ (about half a tonne) of sand.
Watch out: run it once per coat, as scratch, floating and setting coats use different mixes and thicknesses. A trial mix in a bucket is the best check on your own sand.
Splits a coat of lime plaster into lime putty and sand by volume for a chosen mix, such as 1:2.5 scratch or 1:3 floating coat.
Enter your values to see the result.
Good to know
Traditional internal lime plaster: scratch coat about 1:2.5 lime putty:sharp sand, floating coat 1:3, each no more than about 15 mm; setting coat 2–5 mm, often near 1:1 with fine sand. A 1:3 mix works because the putty roughly fills the voids in the sand (about a third of its volume), so the finished volume is close to the sand volume; that is why a factor near 1.33 also suits lime. Run the calculator once per coat.
Common mistake
Ordering by finished volume without the bulking allowance, or using building (soft) sand instead of sharp, well-graded sand for the backing coats.
Limits
Estimating only. Lime putty and sand vary in voids and moisture; a trial mix gauged in a bucket is the reliable check. For NHL (hydraulic lime powder) follow the supplier’s bag coverage instead.
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.