The two steps
volume = area × depth, then weight = volume × density.
Keep everything in meters and the volume comes out in cubic meters, which is how gravel is sold loose. Suppliers who sell by the metric ton need the second step.
A worked example
A path 10 m long and 1.2 m wide, with 50 mm of gravel.
- Area: 10 × 1.2 = 12 m².
- Volume: 12 × 0.05 = 0.6 m³.
- Weight at 1,600 kg/m³: 960 kg, so just under a metric ton.
A driveway 5 m by 3 m at 100 mm is 1.5 m³, or 2.4 metric tons of the same gravel.
How deep
- 50 mm: a decorative bed or a path that is only walked on.
- 75 to 100 mm: a path that takes a wheelbarrow, or gravel over a weed membrane.
- 100 mm or more, over a compacted sub-base: a driveway. The sub-base does the work of carrying the car; the gravel on top is the surface.
- 50 to 75 mm: bark or mulch on a border, deep enough to keep weeds down without smothering the plants.
Densities worth knowing
- Gravel and shingle: about 1,600 kg/m³, so one metric ton covers about 12 m² at 50 mm.
- Sharp sand: about 1,500 kg/m³.
- Topsoil: about 1,300 kg/m³, more when wet.
- Bark and mulch: about 400 kg/m³, which is why a metric ton of bark is a mountain.
- Concrete: about 2,400 kg/m³.
These are dry figures. Wet material is heavier, and every quarry quotes its own density, so use theirs when ordering by weight.
Bags or loose
Small bags are about 20 to 25 kg, so a metric ton is 40 to 50 of them: fine for a border, miserable for a driveway. A bulk bag is usually quoted at around 800 kg, which is half a cubic meter of gravel, and loose by the metric ton is cheapest if you have somewhere to tip it.
Tools
The Gravel Calculator works out the volume, weight and number of bags for a rectangle or a circle, for gravel, sand, soil, mulch or concrete. For a slab use the Concrete Calculator.