Formula
The hole is a cylinder: π times half the hole width squared, times the depth.
The post takes out its own square: post size times post size, times the depth.
Mix volume is the hole minus the post, times the number of posts, times one plus waste. Bags are that volume divided by the bag yield, rounded up once for the whole job.
Worked example
A 300 mm round hole, 600 mm deep, holds 0.0424 m³. A 100 mm post takes 0.0060 m³ of it, leaving 0.0364 m³.
7 posts need 0.255 m³. At 0.01825 m³ a bag that is 14.0 bags, rounded up to 14.
Assumptions
A dug hole is rarely a clean cylinder. A square hole holds about 27% more than a round one of the same width.
UK 20 kg Postcrete starts at 0.01825 m³, a figure worked back from the maker’s table rather than printed on the bag. Australian rapid set uses the printed 0.01 m³ and 0.0151 m³.
The count is pooled across posts. Buying per post, with a part bag rounded up each time, gives a higher number.
Checked
Checked 11 November 2026. The Blue Circle Postcrete data sheet (December 2023) lists 20 kg bags and gives 2 bags for a 100 mm post in a 300 mm × 600 mm hole; that row works back to about 0.0182 m³ a bag, so this page starts at 0.01825. Bunnings lists Easy Mix Rapid Set Concrete at 0.01 m³ a 20 kg bag and 0.0151 m³ a 30 kg bag.