Builders' merchants sell aggregates by the tonne and by the bulk bag, but a job is measured in square metres and millimetres. The bridge is density: how much one cubic metre of the loose material weighs.
Loose density
Baglist uses 1.6 tonnes per cubic metre for gravel and 1.4 for topsoil. Sharp sand at about 1.5 and ballast at about 1.65 are worked back from Wickes, which lists 800 kg jumbo bags covering about 10.6 m² and 9.7 m² at 50 mm.
These are loose, dry figures. Wet sand is heavier, and a sub-base weighs more once it is compacted. If the supplier prints a density, use theirs.
| Material | t/m³ | 800 kg bag at 50 mm | 800 kg bag at 100 mm | 1 t bag at 50 mm |
|---|---|---|---|---|
| Gravel | 1.6 | 10.0 m² | 5.0 m² | 12.5 m² |
| Sharp sand | 1.5 | 10.7 m² | 5.3 m² | 13.3 m² |
| Ballast | 1.65 | 9.7 m² | 4.8 m² | 12.1 m² |
| Topsoil | 1.4 | 11.4 m² | 5.7 m² | 14.3 m² |
From area to tonnes
Multiply length, width and depth in metres to get cubic metres, add a margin, then multiply by the density.
A 5 m by 4 m area of gravel at 100 mm is 2 m³, or 2.2 m³ with 10% added. At 1.6 t/m³ that is 3.52 tonnes: five 800 kg bags in the UK, or four 1 tonne bags in Australia.
Bulk bag sizes
Wickes lists its jumbo bags at 800 kg. Older UK catalogues list 850 kg. Bunnings Trade lists 1 tonne bags in Australia. A bag sold by volume rather than weight needs the volume, not the density.
Small bags
For small bags of decorative stone, the Gravel calculator gives the count in 20 kg or 25 kg bags. For topsoil sold by the litre, use the Topsoil calculator.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.