Render and lime
Sand and cement render: cement bags and sand tonnes
- where
- wet render volume (m³)
- area to render (m²)
- render thickness (mm)
- waste allowance (%)
- dry material volume (m³)
- dry-volume factor, dry materials ÷ wet render
- cement volume (m³)
- cement parts in the mix
- sand parts in the mix
- sand volume (m³)
- cement mass (kg)
- cement bulk density (kg/m³)
- whole cement bags to buy
- cement bag weight (kg)
- sand mass (tonnes)
- sand bulk density (kg/m³)
Explain this equation
This turns an area of sand and cement render into bags of cement and tonnes of sand for a given mix by volume, such as 1:4 or 1:5. Use it when ordering for an external render or internal sand and cement backing coat.
V is the finished (wet) volume: area times thickness, with the 1000 turning millimetres into metres. k converts that to the larger volume of loose dry materials. The mix parts c and s split the dry volume into cement and sand, and the bulk densities ρc and ρs turn volumes into weights.
Dry materials shrink in volume when mixed with water because the cement and water fill the gaps between sand grains, so you need about a third more loose material than the render you end up with; hence k of about 1.33. Cement is then rounded up to whole bags.
Sense check: 100 m² at 15 mm in 1:4 is 1.5 m³ of render, which needs about 23 bags of cement and about 2.6 tonnes of sand. Double the thickness and every quantity doubles.
Watch out: damp sand bulks up and weighs more per cubic metre than dry sand, so check the density your merchant uses when buying by the tonne.
Turns an area and thickness of sand and cement render into cement bags and sand tonnes for a mix ratio by volume.
Enter your values to see the result.
Good to know
The dry-volume factor (commonly 1.27–1.33) allows for the shrinkage in volume when water is added and the cement fills the voids between sand grains; 1.33 is the usual estimating figure. Typical loose bulk densities: cement 1440 kg/m³, building sand 1550–1650 kg/m³ (damp sand bulks and weighs differently). Common UK render mixes are around 1:4 to 1:6 cement:sand, or cement:lime:sand to the BS EN 13914-1 National Annex designations; check the specification for the background and exposure.
Common mistake
Working out the wet volume only and ordering that much sand: you need roughly a third more dry material than the finished render volume.
Limits
An estimating method. Real yield depends on sand grading, moisture and bulking, and on how much is lost to backgrounds and droppings. For lime or plasticiser in the mix, include the lime in the sand parts only if it is a premixed lime:sand.
Sources
- JK Cement, How do you calculate plastering ratio? (dry volume = wet volume × 1.33; cement 1440 kg/m³, sand 1550–1650 kg/m³; worked example)
- MPA Mortar, Learning Text LT12 Rendering mortar (mix designations per BS EN 13914-1 National Annex)
- Mortar Industry Association / MPA, Mortar Data Sheet 04: Rendering (undercoat 10–12 mm, finishing coat 8–10 mm)
- BS EN 13914-1:2016 Design, preparation and application of external rendering and internal plastering. External rendering (National Annex Tables NA.3, NA.4), as summarised by Premier Guarantee, Site made renders
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.