Formula
Volume is internal length times internal width times soil depth, all in metres.
Waste starts at zero because the inside of the box is the volume you fill. You can still add some.
Litres are the volume times 1000. Bags are the litres divided by the bag size, rounded up.
Worked example
The inside of the bed, 2 m by 1 m by 0.4 m, is 0.800 m³.
Waste is 0%, so the fill is 800 litres, 16.0 bags before rounding, exact at 50 L.
Assumptions
The example bed is 2.0 × 1.0 m inside, filled 0.40 m deep, with no waste.
Bags start at 50 L. There is no bulk-bag volume invented here.
Checked
Checked 10 November 2026. B&Q lists Verve bark at 50 L, and Bunnings lists several 50 L mulch bags. The fill is whatever volume is printed on the bag you buy.