The site thinks in volume (m³); the weighbridge measures in weight (tonnes). The bridge between the two is the material’s bulk density — and that value varies with the material, its particle size, its moisture and how tightly the pile is packed.
The logic of the conversion
The weight of a cubic metre of aggregate is not a fixed number. Even material from the same quarry weighs differently in a loose pile and in a compacted state. The calculation therefore rests on three inputs:
- Bulk density: the density of the material in its loose or compacted state.
- Moisture: wet material weighs more, but its volume does not change.
- Bulking / compaction allowance: material placed and compacted takes up less space than its volume on the truck.
The right approach is to calculate the order from the compacted in-place volume required, then confirm with the producer the conversion factor that applies to that job. The value varies by fraction; sand and coarse crushed stone do not give the same number.
A practical calculation sequence on site
- Determine the area to be covered and the compacted layer thickness: area × thickness = in-place volume.
- Add the compaction allowance: the loose volume arriving on site is greater than the in-place volume.
- Convert to tonnage using the conversion factor for that fraction.
- Divide by truck capacity to get the number of loads.
- Leave a margin for losses: edge losses, trimming and measurement tolerance are real.
Haulage and delivery plan
Material being ready at the quarry and material being on site on time are two different things. An order should not be closed until these points have been discussed in the dispatch plan:
- Access: road width, gradient and the truck’s turning space. A large truck cannot tip on a narrow plot.
- Tipping point: avoid handling the material twice; the pile should be close to where it will be used.
- Load interval: a truck waiting on site and a crew without material are both a cost.
- Weighbridge ticket: the record of delivered weight is kept for both payment certificates and traceability.
- Seasonal demand: regional demand rises in the summer months; confirm the schedule before committing to a date.
Stockpiling on site
If piles get mixed, the advantage of the fraction is lost. Different sizes are stockpiled separately on a clean surface; material contaminated with soil causes problems in concrete and in base layers. Pile height is not unlimited either: in a tall pile the coarse particles roll to the edges and the material segregates.
