The formula
tonnes = area × depth × density ÷ 1,000
Where it comes from
Gravel is ordered by the tonne because it is sold by weight, but measured in cubic metres because what you are covering is a volume. The bridge between the two is bulk density, which for gravel runs at around 1,500 kg per cubic metre. Watch that figure: it is not the density of the stone, which is over 2,600, but of the loose material with the voids left between the grains. That is why a cubic metre of gravel weighs a little over a tonne and a half rather than nearly three.
How to work it out by hand
- Multiply the area by the depth in metres
- Add the allowance: gravel settles and compacts once walked on
- Multiply by the density of the material
- Divide by a thousand for the tonnes
What is worth knowing
Depth matters more than it looks. A drive taking cars needs at least 10 cm of gravel, and below 5 cm no layer forms at all: the stone parts and the ground shows through. On soft ground it is worth compacting a hardcore base first and laying a geotextile membrane underneath, which stops the gravel from sinking and mixing with mud. Crushed hardcore weighs considerably more than gravel — around 1,800 kg/m³ — because it has fines filling the voids, and that is why it compacts and gravel does not.