Every fencing job comes down to the same question: bags of postcrete or a proper delivered mix? For two or three posts, bags win on convenience. Beyond that, the maths — and the strength — swing quickly the other way.
The volume maths
Hole volume minus post volume. For the standard UK spec (300mm diameter hole × 600mm deep, 100mm post):
- Per post: ~0.035m³ (≈ 1.5 bags of postcrete)
- 10 posts: ~0.35m³
- 25-post run (a typical garden boundary): ~0.9m³
- Gate posts: double the hole size — 0.08–0.1m³ each
Postcrete vs delivered concrete
- Cost: postcrete averages £6–£7 per bag — a 25-post run costs £230–£260 in bags. The same volume of delivered concrete is about £113 with no part-load fee
- Strength: C25 is roughly twice the compressive strength of set postcrete — it matters for gates and exposed runs
- Speed on big runs: filling holes from a barrow beats splitting and soaking 40 bags one at a time
- Convenience on tiny jobs: 1–3 posts? Just use bags — no delivery beats that
Doing it right on the day
- Dig holes three times the post width, flare the bottom slightly (a “bell” resists heave)
- Add 50mm of gravel at the base so the post end can drain — posts rot from the bottom up
- Brace each post plumb both ways before pouring
- Slope the concrete collar away from the post at ground level to shed water
- Leave 24–48 hours before hanging panels on a delivered mix
Fencing contractors: run the numbers
If you install fencing for a living, a volumetric drop at your first job of the day beats a van full of bags every time — fresher mix, faster holes, stronger fences and a better margin. We offer trade accounts with priority slots. Price up your next run on the instant quote page or call 01924 910 129.
FAQs
How much concrete do I need per fence post?+
A standard 100mm post in a 300mm diameter hole, 600mm deep, needs about 0.035m³ — roughly one and a half bags of postcrete. Ten posts need around 0.3–0.4m³.
Is postcrete as strong as real concrete?+
Postcrete sets fast but typically reaches only 10–15 N/mm² — half the strength of C25. For gate posts, exposed runs or heavy close-board fencing, a delivered C20–C25 mix holds far better in wind.
How deep should fence posts be concreted?+
A quarter of the post height as a minimum — so 600mm deep for a 1.8m (6ft) fence. In exposed or soft ground go to 750mm.
How long before a concreted fence post can take load?+
Postcrete: hang panels after 1–2 hours. Standard concrete: brace the posts and leave 24–48 hours before fixing panels, with full strength at 28 days.