How Much Concrete for Fence Posts?

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): ~1–2 bags per post
• Standard hole (300mm × 600mm): 2 bags per post (3 for fences 2m or taller)
• Large hole (350mm × 750mm): ~3 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.

What Is the Concrete Mix Ratio for Fence Posts?

For hand-mixed fence-post concrete, use 1:2:4 by volume(1 part cement, 2 parts sharp sand, 4 parts 20mm aggregate) — roughly a C20 general-purpose mix — or about 1 part cement to 4–5 parts all-in ballast. Site-batched 1:2:4 is reliable for fence-post footings, paths and shed bases, but keep the mix stiff rather than sloppy, as excess water weakens the set concrete. If you use bagged postcrete instead, no mixing ratio applies: part-fill the hole with water as the bag instructions direct, pour the dry mix in around the braced post, and it sets in 5–10 minutes.

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.