Home & Construction⏱ 4 min read
How Much Concrete Do You Need to Set Fence Posts?
The right amount of concrete per post depends on post diameter, hole depth, and soil type. Here's the exact calculation — and why most DIY guides significantly underestimate what you need.
Using too little concrete for fence posts is one of the most common reasons fences lean and fail within a few years. Here's how to calculate accurately and get it right first time.
The Cylinder Volume Formula
Volume of concrete per hole = pi x r^2 x depth
(Cylindrical hole, r = radius of hole)
But subtract the post volume:
Post volume = post width^2 x depth (for square posts)
or pi x r_post^2 x depth (for round posts)
Net concrete volume = Hole volume - Post volume
Worked Example: Standard 100mm Square Post
Hole: 200mm (8 inch) diameter, 600mm deep
Hole radius: 0.10m
Hole volume = pi x 0.10^2 x 0.6 = 3.14159 x 0.01 x 0.6 = 0.01885 m3
Post (100mm square, 600mm deep in ground):
Post volume = 0.1 x 0.1 x 0.6 = 0.006 m3
Net concrete = 0.01885 - 0.006 = 0.01285 m3 per post
In 25kg bags (one bag ≈ 0.012 m3 when mixed):
Bags per post = 0.01285 / 0.012 = 1.07
→ Use 2 bags per post (always round up — better to have a little left over)
Post Hole Size Recommendations
Post SizeRecommended Hole DiameterBags per Post
75mm round or 75mm square150mm (6")1-2 bags
100mm square200mm (8")2 bags
125mm square250mm (10")2-3 bags
150mm square300mm (12")3 bags
Hole Depth by Fence Height
Minimum hole depth = 1/3 of total post length
Or: fence height / 2 for exposed sites
1.83m (6ft) fence, post needs 0.6m in ground (min):
Total post length: 1.83 + 0.6 = 2.43m → buy 2.4m posts
Sandy/loose soil: increase depth by 150mm (add one extra bag)
Clay soil: standard depth usually sufficient
With concrete, posts in clay may actually need drainage holes
punched in the concrete or a gravel layer at the base to prevent
waterlogging, which accelerates post rot.
Total Bags for a Complete Fence Run
Number of posts = (Run length / panel width) + 1
Example: 20m fence run, 1.83m panels
Posts = (20 / 1.83) + 1 = 10.93 + 1 = ~12 posts
(Always round up post count, not down)
At 2 bags per post: 12 x 2 = 24 bags
Add 10% contingency: 27 bags
Post mix or sharp sand + cement (1:4 ratio)?
Pre-mixed post-fix bags (Postcrete/similar) are easier for small
quantities but cost roughly 3x more per bag than mixing your own.
For 12+ posts, mixing your own is worth the effort.