Formula
Net volume is length times width times depth. Depth in millimetres is divided by 1000.
Volume to buy is the net volume multiplied by one plus the waste percentage. Litres are that volume times 1000, then divided into bags.
Mass is the buy volume times the loose density. It is a second line, not the shopping count.
Worked example
3 m by 2 m at 150 mm is 0.900 m³ net and 0.990 m³ to buy.
That is 990 litres, 39.6 bags, rounded up to 40 at 25 L, and 1.386 tonne at 1.4 t/m³.
Assumptions
The default bag is 25 L. Bulk does not invent a cubic metre size for a bulk bag.
1.4 tonne per cubic metre is a loose topsoil assumption. Compacted soil is heavier. Change the density if your supplier prints one.
Checked
Checked 10 November 2026. Wickes, B&Q Verve and Bunnings Garden Basics all list 25 L bags. The 1.4 t/m³ loose density is still an assumption, not a printed bag weight.