Gravel is sold by weight, and a path or a border is measured by area and depth. The bridge between them is density: how many tonnes one cubic metre of loose stone weighs.
The density assumption
Baglist starts at 1.6 tonnes per cubic metre for loose decorative stone. It is an assumption, not a printed figure, and a supplier who prints their own should be believed over it.
At 1.6 t/m³, one square metre at 50 mm deep is 0.05 m³, which weighs 80 kg. That is 3.2 bags of 25 kg, or 4 bags of 20 kg.
| Depth | kg per m² | 25 kg bags | 20 kg bags |
|---|---|---|---|
| 30 mm | 48 | 1.92 | 2.4 |
| 50 mm | 80 | 3.2 | 4 |
| 75 mm | 120 | 4.8 | 6 |
Turning it round
A 25 kg bag at 1.6 t/m³ is 0.0156 m³. At 50 mm deep it covers about 0.31 m². A 20 kg bag covers 0.25 m² at the same depth.
That is less than many people expect. It is why a path is usually cheaper to fill from a bulk bag than from small bags, and why it is worth checking the bulk bag option on the Tonnes calculator.
A worked path
A 5 m by 2 m area at 50 mm is 0.5 m³. With 10% waste it is 0.55 m³, which weighs 880 kg. In 25 kg bags that is 35.2, so 36. In Australian 20 kg bags it is 44 exactly.
Choosing a depth
Depth is a choice, not a rule. Thin layers show the membrane underneath as the stone moves; deep layers are hard to walk on. Follow the advice on the bag for the stone you are buying.
This page is for decorative stone over a prepared base. It is not a specification for a structural sub-base, which is compacted and weighs more per cubic metre.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.