How it works
Concrete shrinks as the water fills the gaps between stones, so the dry materials needed are about 1.54 times the finished (wet) volume. That dry volume is shared by the mix ratio: in 1:2:4, ballast is 4 parts out of 7. Ballast weighs about 1.5 tonnes per m³ and sand about 1.6. For fill, the tonnage is simply the area × depth × 1.5.
Step by step
- Multiply length × width × thickness (in metres) for the wet volume of concrete.
- Multiply by 1.54 for the dry volume of materials.
- Take the ballast share of the mix (4/7 for 1:2:4) and multiply by 1.5 t/m³ for tonnes.
- Add wastage and divide by the lorry size, rounding up to whole loads.
Worked example
A 6 × 4 m slab, 150 mm thick, in 1:2:4: wet volume 3.6 m³, dry volume 5.54 m³. Ballast = 5.54 × 4/7 × 1.5 = 4.75 tonnes, plus 5% = 5 tonnes, which is one 7-tonne lorry. The same slab also needs about 24 bags of cement and 2.7 tonnes of sand.
Questions
How many tonnes of ballast are in a cubic metre?
About 1.5 tonnes per m³ of loose ballast.
How much ballast does a lorry carry?
Our ballast is sold by the lorry load (the size is in the product name). The calculator rounds up to whole loads; for a smaller load or a site with tight access, ask us for a smaller lorry.
What does 1:2:4 mean?
One part cement, two parts sand and four parts ballast by volume. It gives roughly C20 concrete, used for most slabs and beams.