How Much Concrete for Fence Posts?
Quick Answer
~1–2 bags (20kg postcrete) per post, or 0.03m³ concrete per post
Based on: 300mm diameter hole, 600mm deep, 100mm square post centred in the hole
How We Calculated This
The concrete fills the hole around the post. Calculate the hole volume minus the post volume:
- Hole diameter: 300mm (use a post-hole digger or auger)
- Hole depth: 600mm
- Hole volume: π × 0.15² × 0.6 = 0.042m³
- Post volume: 0.1 × 0.1 × 0.6 = 0.006m³
- Concrete needed: 0.042 − 0.006 = ~0.036m³ per post
- In postcrete: ~1.5 bags of 20kg per post
Quick Reference: Posts per 20kg Bag of Postcrete
- Small hole (250mm × 500mm): ~1 bag per post
- Standard hole (300mm × 600mm): ~1.5 bags per post
- Large hole (350mm × 750mm): ~2.5 bags per post
Postcrete vs Mixed Concrete
- Postcrete (e.g., Post Mix): Just add water, sets in 5–10 minutes. Convenient but more expensive per post (~£4–6 per bag).
- Ready-mix concrete: Mix ballast, cement, and water. Cheaper for many posts but takes 24–48 hours to set.
- Mix ratio for fence posts: 1 cement : 4 ballast (or use a general-purpose mix)
Example: 10m Fence Run
- Panels: 6 (at 1.83m each)
- Posts: 7
- Postcrete: 7 × 1.5 = ~11 bags (20kg)
- Or mixed concrete: 7 × 0.036 = ~0.25m³
Tips
Dig holes 3 times the width of the post and at least 600mm deep. Drop 50mm of gravel in the bottom for drainage. Set the post plumb and brace it while the concrete sets. Slope the top of the concrete away from the post to shed rainwater. In clay soil, make holes slightly wider as clay holds water around the post base.