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 (compacted) | 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 |
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.
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
- Bulk densities are typical loose-bulk values published on Australian and New Zealand landscape-supply and quarry price lists (commonly 1.4–1.6 t/m³ for gravels and sands, 1.7–1.8 t/m³ for compacted road base, 0.9–1.1 t/m³ for scoria). Suppliers' figures vary; 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 23 September 2026 · Report an error