Formula
Buy volume is length times width times depth, then multiplied by one plus the waste percentage.
Mass in kilograms is that volume times the loose density in tonnes per cubic metre, times 1000.
Bag count is the mass divided by the bag weight, rounded up.
Worked example
5 m by 2 m at 50 mm is 0.500 m³ net and 0.550 m³ with 10% waste.
At 1.6 t/m³ that is 880 kg, 35.2 bags, rounded up to 36 at 25 kg.
Assumptions
The default loose density is 1.6 tonne per cubic metre. It is a bulk assumption for decorative stone, not a specification for a structural sub-base.
UK bags start at 25 kg. Australian bags start at 20 kg. Both weights are on the page.
Checked
Checked 10 November 2026. Bunnings lists Ki-Carma drainage gravel and river pebbles at 20 kg. Wickes sells decorative gravel in major bags, and that category page did not show a 25 kg label, so change the bag weight if yours differs. The 1.6 t/m³ loose density is still an assumption.