An under-concreted fence post is the most common cause of fence failures in the UK. One winter storm later, the panel is flat on the ground. The fix requires digging out cured concrete and starting again — far more effort than getting it right first time. Here's exactly how to calculate what you need.
How Deep Should Fence Post Holes Be?
The standard rule: posts should be buried one third of their total length, with a minimum of 600 mm (2 feet) regardless. For exposed/coastal sites or heavy panels, increase to 45–50%.
| Fence Height | Post Length | Hole Depth | Above Ground |
|---|---|---|---|
| 0.9 m (3ft) | 1.5 m (5ft) | 600 mm (2ft) | 900 mm |
| 1.2 m (4ft) | 1.8 m (6ft) | 600 mm (2ft) | 1,200 mm |
| 1.5 m (5ft) | 2.1 m (7ft) | 700 mm (2ft 4in) | 1,400 mm |
| 1.8 m (6ft) | 2.4 m (8ft) | 750 mm (2ft 6in) | 1,650 mm |
Calculating Hole Volume
Hole volume (m³) = π × radius² × depth (circular hole)
For a 300 mm diameter × 750 mm deep hole:
V = π × 0.15² × 0.75 = π × 0.0225 × 0.75 = 0.053 m³ per hole
Subtract post volume if post is large: π × post_radius² × hole_depth
Concrete Per Post — Quick Reference
| Hole diameter | Hole depth | Volume (m³) | Postcrete bags (20kg) |
|---|---|---|---|
| 200 mm | 600 mm | 0.019 | 1 bag |
| 250 mm | 600 mm | 0.029 | 1–2 bags |
| 300 mm | 750 mm | 0.053 | 2–3 bags |
| 350 mm | 750 mm | 0.072 | 3 bags |
| 400 mm | 900 mm | 0.113 | 5 bags |
One 20kg bag of Postcrete sets in approximately 3 minutes and yields around 11 litres (0.011 m³). One 25kg bag of traditional concrete mix (pre-bagged) yields approximately 12.5 litres.
Postcrete vs Traditional Mix — Which Should You Use?
| Postcrete / Fast Set | Traditional Mixed Concrete | |
|---|---|---|
| Setting time | 3–5 minutes | 2–24 hours (working time 30–60 min) |
| Ease | Pour dry into hole, add water | Mix in barrow or mixer |
| Cost per hole | £4–6/bag × 2 bags = £8–12 | £3–5 equivalent |
| Strength | C20 equivalent | C20–C25 depending on mix |
| Best for | DIY, up to 20 posts | Large quantities, better value |
Step-by-Step Process
- Mark post positions with string line — check spacing and alignment before digging
- Dig holes using a post-hole borer (rents for £50–80/day) — much faster than manual methods for hard ground
- Add 100 mm drainage gravel at the base of each hole to prevent water pooling
- Set post, check plumb — use a post level (£5 tool) on two sides simultaneously; brace if needed
- Pour Postcrete dry, add water per bag instructions — do not over-water
- Form a collar above ground: angle the concrete away from the post at 45° to shed rainwater
- Leave undisturbed for 24 hours before loading with fence panels