My brother-in-law set a run of 14 fence posts last year and ran out of Postcrete with three posts to go. He had done a rough calculation based on one bag per post (he had seen that figure somewhere online) and bought 14 bags. The figure of "one bag per post" assumes a very small hole with a very small post. His holes were 300mm diameter and 700mm deep. He needed closer to four or five bags per post.
He ended up improvising with a bag of standard dry-mix concrete from the shed for the last few, and you can honestly see where those three posts are in the fence line - they are the ones that are very slightly different.
The right figure depends on your hole size and depth. Here is how to work it out.
Three types of concrete for fence posts
Postcrete / Post-mix (fast-setting dry compound) Pour dry into the hole around the post, add water. Sets in approximately 10-15 minutes. Ideal for runs of up to 20-30 posts. More expensive per post than mixing concrete yourself, but enormously faster and simpler. No mixer needed, no mixing required.
Dry-mix concrete bags (20kg) Standard pre-mixed concrete bags that you add water to and mix yourself. Cheaper per bag than Postcrete, but you need a mixer or a strong back and a large bucket. Better value for 30+ posts where the material saving justifies the extra work.
Ready-mix concrete Delivered by truck, ordered in m3. Economical for 50+ posts or if you are combining with other concrete pours on site. Minimum orders apply (often 1-2m3), so small projects may not qualify.
How to calculate concrete per hole
The net concrete volume is the hole volume minus the post volume. Here is the formula:
Round hole volume: pi x (diameter/2)2 x depth
Square post volume: side x side x depth
Net concrete: Hole volume - Post volume
Bags: Net volume / Bag yield (0.010 m3 for a standard 20kg bag)
Worked example: standard fence post
- Hole: 250mm diameter, 600mm deep
- Post: 100mm square
Hole volume = 3.14159 x (0.125)2 x 0.6 = 0.0295 m3
Post volume = 0.1 x 0.1 x 0.6 = 0.006 m3
Net concrete = 0.0295 - 0.006 = 0.0235 m3
Bags = 0.0235 / 0.010 = 2.35 - round up to 3 bags
For 14 posts: 14 x 3 = 42 bags - not 14.
Use the Fence Post Concrete Calculator or Postcrete Calculator to calculate the total for your whole project.
Bags per post by hole and post size
100mm square post, various hole sizes:
| Hole diameter | Hole depth | Bags per post |
|---|---|---|
| 200mm | 500mm | 2 bags |
| 200mm | 600mm | 2 bags |
| 250mm | 600mm | 3 bags |
| 250mm | 750mm | 3-4 bags |
| 300mm | 600mm | 4 bags |
| 300mm | 750mm | 5 bags |
Based on 20kg bags yielding 0.010 m3.
Hole depth guidance
The standard rule for fence posts is one third of the total post length in the ground:
| Fence height | Post length | Hole depth |
|---|---|---|
| 0.9m (3ft) | 1.5m | 500mm |
| 1.2m (4ft) | 1.8m | 600mm |
| 1.5m (5ft) | 2.1m | 700mm |
| 1.8m (6ft) | 2.4m | 800mm |
| 2.1m (7ft) | 2.7m | 900mm |
In exposed or windy locations, or in sandy/loose soil, go deeper than the minimum. A post that blows out of the ground three years after installation costs far more to fix than the extra 100mm of depth would have.
Hole diameter guidance
The hole should be roughly 2.5-3x the post width in diameter to give the concrete enough contact area with the ground:
| Post size | Minimum hole diameter |
|---|---|
| 75mm post | 200mm |
| 100mm post | 250mm |
| 125mm post | 300mm |
| 150mm post | 350mm |
Undersized holes create a thin cylinder of concrete that can be levered out over time by the post flexing in wind. Width matters as much as depth.
When does ready-mix become worth it?
For very large fence projects, ready-mix starts to make economic sense. The crossover point depends on the size of your holes:
At 0.024 m3 per post (standard 100mm post, 250mm x 600mm hole):
- 30 posts = 0.72 m3 concrete
- 50 posts = 1.2 m3 concrete
Most ready-mix suppliers have minimum orders of 1-2 m3. For 50+ posts, ready-mix ordered alongside other site work often makes sense. For smaller runs, Postcrete or dry-mix bags are simpler.
My tips for setting fence posts in concrete
After setting more posts than I would like to count, including fixing several that previous owners had set incorrectly:
Use a post hole borer. A two-person manual post hole borer (or a hired petrol borer) produces clean, round holes much faster than a spade or crowbar. The right-sized round hole makes calculating concrete quantities more accurate too.
Check for underground services first. Before boring, call 0800 96 73 72 (Dial Before You Dig in the UK) or check your own utility maps. Hitting a cable or pipe while boring is expensive and dangerous.
Brace posts plumb during the pour. Use two pieces of timber nailed diagonally to the post and pegged into the ground to hold it exactly vertical. Remove them after 24 hours. A post that sets 2 degrees out of plumb is very obvious once the panels are on.
Finish the concrete proud of the surface, then slope it. When the concrete has set but is still workable (around 45-60 minutes for Postcrete), use a trowel to create a slight downward slope away from the post face on all sides. This sheds rainwater away from the post base - the most common point of rot.
Treat post bases before setting. If using timber posts, soak the underground section in preservative before setting them. This is worth doing even with pressure-treated timber.
For the full bag count across your project, use the Fence Post Concrete Calculator.