Why gravel is sold by the tonne, and how to convert
Landscape supply yards weigh trucks on a weighbridge, so bulk gravel, sand and crushed rock are priced per tonne. Your project, though, is measured in square metres and millimetres. The bridge between the two is bulk density – the weight of one loose cubic metre:
| Material | Typical loose density | 1 t covers at 50 mm | Common uses |
|---|---|---|---|
| 20 mm blue metal / drainage gravel | 1.5 t/m³ | 13.3 m² | Drainage, driveways, under concrete |
| 10 mm gravel, pea gravel | 1.5 t/m³ | 13.3 m² | Paths, pot drainage, decorative |
| Decorative pebbles, river stone | 1.5–1.6 t/m³ | 12.5–13 m² | Garden beds, water features |
| Crusher dust / cracker dust | 1.6 t/m³ | 12.5 m² | Synthetic turf base; paver bedding only where the paver maker allows it (see the paving guide) |
| Road base (loose) | 1.7–1.8 t/m³ | 11–12 m² | Driveway and path base layers |
| Scoria | 0.9–1.1 t/m³ | 18–22 m² | Lightweight drainage, wicking beds, rooftops |
| Sand (paving, brickies) | 1.4–1.6 t/m³ | 12.5–14 m² | Bedding pavers, mortar, top-dressing |
Buying by the tonne but measuring in cubic metres, or the other way round? The cubic metres to tonnes calculator converts for 12 materials, and compares these densities with a Sydney supplier's published figures.
Density is also why half a cubic metre of scoria is a comfortable trailer load and the same volume of road base is not. Most 6×4 trailers are rated at 750 kg including the trailer's own 200–275 kg, which leaves about 500 kg. That is only a third of a cubic metre of gravel, and many yards will refuse to load more. The trailer load calculator works it out for your trailer.
Worked example: a gravel path
A 12 m long, 1 m wide garden path with a 50 mm road base layer and 30 mm of decorative 10 mm gravel on top:
- Area: 12 × 1 = 12 m².
- Road base: 12 × 0.05 = 0.6 m³ × 1.7 = 1.02 t. Add 10 % for compaction → 1.1 t.
- Gravel topping: 12 × 0.03 = 0.36 m³ × 1.5 = 0.54 t.
- Order 1.1 t road base and 0.55 t gravel (yards usually sell in 0.25 t or 0.5 t increments). In 20 kg bags that would be 55 + 27 = 82 bags – a clear case for bulk delivery.
Setting stepping stones in pebbles instead? The stepping stone calculator works out the stones, the gaps and the pebbles between them together.
How deep should gravel be?
- Decorative top layer on beds: 30–50 mm over weed mat or a compacted base. Deeper looks no better and is harder to walk on.
- Garden path: 50–75 mm road base compacted, then 30–40 mm of 10–14 mm gravel. Smaller stone locks together and is kinder underfoot.
- Driveway: 100–150 mm compacted road base, topped with 50 mm of 20 mm gravel. Use the compaction checkbox.
- Drainage behind a retaining wall: a 300 mm wide column of 20 mm blue metal for the full height – calculate it as a rectangle 0.3 m wide by the wall length, with the wall height as "depth".
- Drainage layer in a raised bed on concrete: 30–50 mm of scoria or 10 mm gravel, and no more – the raised bed calculator handles this as a layer.
Gravel questions
How many square metres does a tonne of gravel cover?
At 1.5 t/m³ a tonne is 0.67 m³, which covers about 13 m² at 50 mm, 22 m² at 30 mm or 6.7 m² at 100 mm.
How many 20 kg bags of gravel in a cubic metre?
About 75 bags for gravel at 1.5 t/m³ (1,500 kg ÷ 20 kg). Bagged gravel becomes expensive very quickly – over about 0.3 m³ a bulk delivery is almost always cheaper.
Do I need weed mat under gravel?
Under decorative gravel on a garden bed, yes – a woven geotextile stops the stone sinking into the soil and slows weeds. Under a compacted path or driveway base, geotextile is used for separation on soft ground rather than for weeds.
Sources and method
- Australian Native Landscapes, Landscape product weights and volumes – loose densities: blue metal 1.45, river gravel 1.55, other decorative gravels 1.5, granite 1.6, sands 1.3–1.5, road base 1.75 (2.4 compacted), road gravel 1.8, scoria 1.0 t/m³. Suppliers' figures vary with grade and moisture; use theirs when quoted.
- Depth guidance reflects standard landscaping practice for paths and driveways; check local council requirements for driveways and crossovers.
- Volume and weight conversions are geometry and multiplication only; bag counts round up.
Written by DueRiva · Last reviewed 29 September 2026 · Report an error