Formula
Net volume is length times width times thickness. Thickness is entered in millimetres and divided by 1000.
The volume you buy is the net volume multiplied by one plus the waste percentage.
Bag count is that volume divided by the yield of one bag, then rounded up. An exact division does not add another bag.
Worked example
A 4 m by 2.5 m slab, 100 mm thick, is 1.000 m³ net.
With 10% waste the buy is 1.100 m³. At 0.010 m³ a bag that is 110.0 bags before rounding, exact.
Assumptions
UK 20 kg ready-mix starts at 0.010 m³, from the Wickes Blue Circle listing. UK 25 kg starts at 0.010 m³, from the Concrete Society example.
Australia 20 kg starts at 0.010 m³, 25 kg at 0.012 m³, and 40 kg at 0.019 m³.
The box you enter is treated as a solid rectangle. The bag yield is not. Change it to the number printed on the bag.
Checked
Checked 10 November 2026. Wickes lists Blue Circle Multi-Purpose Ready To Use Concrete, 20 kg, at about 0.01 m³. The 25 kg figure is the Concrete Society example of 0.010 m³, and that page says the printing on the bag wins. The Cemix Multicrete datasheet of February 2023, also quoted on the Bunnings NZ 20 kg listing, gives Australia 20 kg at 0.01 m³, 25 kg at 0.012 m³ and 40 kg at 0.019 m³.