About this calculator
How many bags of rapid-set concrete or concrete mix you need to set fence posts, mailbox posts or pergola legs. The post’s own volume is subtracted.
How it’s calculated
Concrete per hole = (hole area − post area) × (hole depth − gravel depth). Multiply by the number of posts, add waste, and divide by the bag yield.
Worked example
With these inputs:
- Number of posts
- 10
- Hole diameter
- 300 mm
- Hole depth
- 600 mm
- Post shape
- Square
- Post size
- 100 mm
- Gravel at base
- 100 mm
- Product
- Rapid-set concrete 20 kg
- Waste allowance
- 10%
the result is:
- Bags needed
- 36 bags
- Concrete per hole
- 30 L
- Total concrete
- 0.33 m³
- Gravel for bases
- 71 L
Tips from the trade
- Hole diameter is usually about three times the post width.
- Bury about one third of the post’s total length, and go deeper on exposed or windy sites.
- Gravel in the bottom of the hole lets water drain away from the post end.
- Slope the top of the concrete away from the post so water runs off.
Common questions
How many bags per fence post?
A 100 mm post in a 300 mm × 600 mm hole, with 100 mm of gravel in the bottom, takes three to four 20 kg bags of rapid-set concrete.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the NCC and Australian Standards before ordering.