Topsoil is bought in small bags by the litre, or loose and in bulk bags by the tonne or cubic metre. The volume is the same either way; only the unit on the label changes.
Volume first
Volume is length times width times depth. A 3 m by 2 m bed at 150 mm is 0.9 m³. With 10% added for settling and an uneven base, it is 0.99 m³, which is 990 litres.
Wickes, B&Q and Bunnings all list topsoil in 25 L bags. 990 litres is 39.6 bags, so 40.
| Depth | Litres per m² | 25 L bags per m² |
|---|---|---|
| 50 mm | 50 | 2 |
| 100 mm | 100 | 4 |
| 150 mm | 150 | 6 |
| 200 mm | 200 | 8 |
Tonnes
Baglist uses 1.4 tonnes per cubic metre as a loose topsoil density. That makes the 0.99 m³ above about 1.39 tonnes. It is an assumption; wet soil and compacted soil weigh more.
If your supplier sells by the tonne, the Tonnes calculator turns the same area and depth into tonnes and bulk bags.
Small bags or bulk
Forty 25 L bags is a lot of carrying, but it suits a garden with no access for a delivery. Bulk is usually easier above a cubic metre, if there is somewhere for the bag to land.
The calculator does not invent a volume for a bulk bag, because suppliers fill them differently. Ask what is in the bag, in cubic metres or tonnes.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.