A garden fence I looked at had been blown over in a winter storm - the posts had snapped at ground level rather than being pulled out of the ground. That looked like adequate depth on the surface, but when I excavated one of the remaining posts, the concrete base was barely 300mm deep and only 150mm in diameter. On a 1.8m fence in an exposed garden, that was nowhere near enough.
The post snapped rather than being pulled out because the concrete was too shallow and too narrow. The post was trying to rotate at the base, couldn't because of the concrete, and the stress moved up to the weakest point - where the timber met the top of the concrete.
The one-third rule
The standard UK guidance for fence post hole depth is:
Post hole depth = one third of total post length (minimum)
For a 1.8m (6ft) fence, the above-ground height is 1.8m. The post below ground should be at least 600mm (0.6m). Total post length: 2.4m.
| Fence height | Minimum post below ground | Recommended total post length |
|---|---|---|
| 0.9m (3ft) | 300mm | 1.2m |
| 1.2m (4ft) | 400mm | 1.6m |
| 1.5m (5ft) | 500mm | 2.0m |
| 1.8m (6ft) | 600mm | 2.4m |
| 2.1m (7ft) | 700mm | 2.8m |
| 2.4m (8ft) | 800mm | 3.2m |
These are minimum depths. In exposed locations, increase depth by 150-200mm.
Hole diameter
The hole diameter must be wide enough to allow adequate concrete around the post. A rule of thumb is approximately 3 times the post width.
| Post size | Recommended hole diameter |
|---|---|
| 75mm x 75mm (3x3 inch) | 200mm |
| 100mm x 100mm (4x4 inch) | 250-300mm |
| 125mm x 125mm (5x5 inch) | 300-350mm |
A hole that is too narrow allows insufficient concrete around the post, reducing the effective base area and increasing the risk of leaning.
Soil type matters
The standard depths assume reasonable soil conditions. Adjust for your soil:
Sandy or loose soil: Increase depth by 150-200mm. Sandy soil has lower bearing capacity and offers less lateral resistance.
Clay soil: Clay expands when wet and contracts when dry, putting cyclical stress on posts. Ensure concrete extends below the frost line (typically 500mm in UK) and at least 100mm into stable soil below the clay.
Made ground / fill: If the garden has been filled (common in new builds and areas with building history), the fill may be loose and compressible. Increase depth until you reach stable natural ground.
Rocky ground: You may only achieve the depth you can dig. In this case, use the widest hole possible to maximise concrete around the post.
Waterlogged ground: Persistent waterlogging accelerates post rot and can affect the concrete base. Use concrete posts (no timber-to-ground contact) or hot-dip galvanised metal spikes if depth is limited.
Concrete vs postcrete vs alternative fixings
Concrete (mixed): Best for exposed positions, large posts, boundary fences that must last. Use a 1:3:6 mix (cement:sand:coarse aggregate) or a weaker 1:2:4 for standard fence posts. Typically requires 2-3 days before fence panels are hung.
Postcrete (dry-pour): Fast setting, sold in 20kg bags. Pour around the post dry, add water, no mixing required. Initial set in 5-10 minutes; hang panels the same day. Good for standard domestic use.
Post spikes: Driven into ground, post slots into metal socket. Fast but less strong than concreted posts. Not suitable for tall fences, exposed locations or poor soil. The spike must be driven absolutely plumb or the fence will lean.
Note: For 1.8m+ fences in exposed gardens, concrete (either pre-mixed or postcrete) is strongly recommended over post spikes.
My tips on fence post holes
Hire a post hole borer. Digging post holes with a spade is tedious and rarely achieves a clean, consistent hole. A hired manual or petrol post hole borer (also called an earth auger) produces clean holes in a fraction of the time.
Check for services before digging. Services (gas, electricity, water, telecoms) are buried in gardens more often than people realise. Call 0800 96 2300 or use the Dial Before You Dig service before using a post hole borer.
Get the post plumb before the concrete sets. Use a spirit level on two adjacent faces. Pack temporary braces or wedges while the concrete cures - postcrete especially sets fast and mistakes are hard to correct.
Protect timber at ground level. Even pressure-treated timber will eventually rot where it meets the concrete at ground level, because this is where moisture sits. Apply end-grain preserver to all cut ends and to the timber at the concrete/air interface before installation.
Use the Fence Post Concrete Calculator or Postcrete Calculator to work out how much concrete you need for your posts.