About this calculator
How much cement, sand and stone you need to set fence posts, gate 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, then work out the cement, sand and stone at the low-strength (foundation) mix.
Worked example
With these inputs:
- Number of posts
- 10
- Hole diameter
- 250 mm
- Hole depth
- 600 mm
- Post shape
- Square
- Post size
- 75 mm
- Gravel at base
- 100 mm
- Waste allowance
- 10%
the result is:
- Cement needed
- 2 × 50 kg bags
- Concrete per hole
- 22 L
- Total concrete
- 0.24 m³
- Stone for drainage bases
- 49 L
- Cement (50 kg bags)
- 2 bags
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 75 mm post in a 250 mm × 600 mm hole, with 100 mm of stone in the bottom, takes about 22 litres of concrete. At the low-strength mix one 50 kg bag of cement makes about 170 litres, enough for about seven posts.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.