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.
Bag count is the litres divided by the bag size, rounded up. Bulk skips the bag count.
Worked example
4 m by 3 m at 50 mm is 0.600 m³ net.
With 5% waste that is 0.630 m³, 630 litres, and 12.6 bags, rounded up to 13 at 50 L a bag.
Assumptions
50 L, 70 L and 100 L are common pack sizes in both countries. They are not tied to one current shelf label.
Bulk does not assume a volume for a bulk bag. It stops at cubic metres and litres.
Checked
Checked 10 November 2026. B&Q lists Verve bark and mulch at 50 L and 100 L. Bunnings lists Garden Basics pine bark at 50 L and Richgro natural pinebark at 70 L. Bulk stays in cubic metres and has no bag count.