Ordering and mixing
Site mix materials: cement, sand and aggregate
- where
- dry volume of materials (m³)
- wet-to-dry volume factor
- volume of concrete needed (m³)
- total parts in the mix
- cement parts
- sand parts
- coarse aggregate parts
- cement (kg)
- cement bulk density (kg/m³)
- cement bags
- cement bag size (kg)
- sand (t)
- sand loose bulk density (kg/m³)
- coarse aggregate (t)
- coarse aggregate loose bulk density (kg/m³)
Explain this equation
Works out the cement, sand and coarse aggregate for a site-mixed concrete by volume ratio, such as 1:2:4.
Loose dry materials shrink when mixed with water and compacted, because the sand fills the gaps in the stone and the cement fills the gaps in the sand. The wet-to-dry factor f (about 1.54) gives the dry volume needed for each cubic metre of finished concrete. Each material takes its share of the parts, and its bulk density turns that volume into a weight; sand and aggregate are divided by 1000 to give tonnes.
Volume shares by ratio, then mass equals volume times bulk density.
Sense check: 1 m³ of 1:2:4 needs about 320 kg of cement (13 bags of 25 kg), 0.7 t of sand and 1.3 t of 20 mm aggregate.
Watch out: bulk densities vary with moisture and grading. Volume-batched site mixes are less consistent than designed ready-mix, so keep them for non-structural work unless specified.
Works out how much cement, sand and coarse aggregate to buy to mix a given volume of concrete on site by volume ratio, such as 1:2:4.
Enter your values to see the result.
Good to know
Loose dry materials contain air voids that close up when water is added and the concrete is compacted, so about 1.54 m³ of dry materials make 1 m³ of concrete; that is the factor f. The bulk densities are typical loose values: weigh a known volume, or use your merchant’s figure, if you want better accuracy. Sand that is damp bulks up, so measure by the bucket consistently and keep sand covered. For structural work, BS 8500 and BS EN 206 favour a designed or designated ready-mix concrete over a site volume mix.
Common mistake
Adding the ratio parts to the wet volume directly (1 m³ of 1:2:4 is not 1/7 m³ of cement), which under-orders cement by about a third.
Limits
An estimating method for nominal volume-batched mixes, not a mix design. Strength depends on the water content, materials and compaction, which this does not control.
Sources
- Brick&Bolt, How many bags of cement for one cubic metre (wet-to-dry factor 1.54; cement 1440 kg/m³; 1:2:4 worked example)
- Calculate.co.nz, Concrete mix ratio calculator (1 m³ wet concrete needs about 1.54 m³ of dry materials; cement about 1440 kg/m³)
- BS 8500-1:2023 Concrete. Complementary British Standard to BS EN 206. Method of specifying and guidance for the specifier
- BS EN 206:2013+A2:2021 Concrete. Specification, performance, production and conformity
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.