Two bags a post is the rule most people hear. It is a good rule for one size of post in one size of hole, and it drifts quickly once either changes.
Where two bags comes from
The Blue Circle Postcrete data sheet gives 2 bags of 20 kg for a 100 mm post in a hole 300 mm wide and 600 mm deep. Worked back, that is about 0.018 m³ a bag, and Baglist starts the UK bag at 0.01825 m³.
A round hole that size holds about 0.042 m³. The post takes 0.006 m³ of it, leaving 0.036 m³ to fill: two bags.
Bags per post by size
The hole volume grows with the square of its width, so a slightly wider hole takes noticeably more. Depth adds in proportion.
Australian rapid-set bags are smaller by volume. Bunnings lists Easy Mix Rapid Set at 0.01 m³ for 20 kg and 0.0151 m³ for 30 kg.
| Post | Hole | Mix | UK 20 kg | AU 20 kg | AU 30 kg |
|---|---|---|---|---|---|
| 75 mm | 225 × 600 mm | 0.021 m³ | 1.12 | 2.05 | 1.36 |
| 100 mm | 250 × 600 mm | 0.024 m³ | 1.29 | 2.35 | 1.55 |
| 100 mm | 300 × 600 mm | 0.036 m³ | 2.00 | 3.64 | 2.41 |
| 100 mm | 300 × 750 mm | 0.046 m³ | 2.49 | 4.55 | 3.01 |
| 125 mm | 350 × 750 mm | 0.060 m³ | 3.31 | 6.04 | 4.00 |
Count the whole job, then round
Rounding per post buys too much. Seven 100 mm posts in 250 mm holes need 1.29 bags each, 9 bags for the job, but 14 if every post is rounded up to 2 first. The calculator adds the posts together and rounds once.
A square hole holds about 27% more than a round one of the same width. If you dig square, measure it as a wider round hole, or add waste.
Hole size
Blue Circle suggests a hole about three times the width of the post. Easy Mix suggests about twice. A common rule of thumb is to bury about a quarter of the post's length. Follow the instructions on the bag you buy, and the fence maker's guidance for tall or exposed fences.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.