Bury a fence post to one third of its above-ground height, with an absolute minimum of 600mm. A 1.8m fence needs 600mm in the ground, so the post has to be 2.4m long. A 2.4m fence needs 800mm buried and a 3.2m post.
The rule people forget is the hole width. It should be roughly three times the post width, and that matters as much as depth — a deep hole barely wider than the post gives the concrete almost nothing to grip.
Depth by fence height
| Fence height | Buried depth | Post length | Hole diameter |
|---|---|---|---|
| 0.9m (3 ft) | 600mm | 1.5m | 300mm |
| 1.2m (4 ft) | 600mm | 1.8m | 300mm |
| 1.5m (5 ft) | 600mm | 2.1m | 300mm |
| 1.8m (6 ft) | 600mm | 2.4m | 300mm |
| 2.1m (7 ft) | 700mm | 2.8m | 350mm |
| 2.4m (8 ft) | 800mm | 3.2m | 400mm |
Note that 600mm covers everything up to a 1.8m fence, which is the most common domestic height. Below that the minimum governs rather than the one-third rule.
Why width matters as much as depth
A fence post fails by rotating in the ground, not by sinking. Wind pushes the panel, the post pivots about a point roughly two thirds down its buried length, and the top moves.
What resists that rotation is the concrete collar bearing against the surrounding soil. A wider collar spreads the load over more soil and gives far more leverage against rotation.
Doubling the hole diameter from 200mm to 400mm roughly quadruples the bearing area and dramatically increases resistance. Going 100mm deeper achieves much less. If you have to choose, go wider.
The standard guidance is three times the post width: a 100mm post wants a 300mm hole. Many people dig a 150mm hole because it’s easier, and then wonder why the fence leans after two winters.
Concrete per post
Volume is the hole minus the post. For a 300mm round hole 600mm deep with a 100mm square post:
π × 0.15² × 0.6 = 0.042 m³ for the hole, minus 0.1 × 0.1 × 0.6 = 0.006 m³ for the post, giving 0.036 m³ per post.
| Hole size | Depth | Concrete per post | 20 kg postcrete bags |
|---|---|---|---|
| 250mm dia | 600mm | 0.023 m³ | 2–3 |
| 300mm dia | 600mm | 0.036 m³ | 4 |
| 300mm dia | 750mm | 0.047 m³ | 5 |
| 400mm dia | 800mm | 0.092 m³ | 10 |
Multiply by the number of posts and add a spare bag per three posts. Nothing wastes an afternoon like running out with two posts left. The post hole calculator handles round and square posts and gives bag counts for whatever you’re using.
Exposed and windy sites
Wind load is the whole reason posts fail, and it scales with the panel area. An open field boundary or a coastal site sees far more of it than a fence between two houses.
On an exposed site, go one step further than the table: an extra 100 to 150mm of depth, a wider hole, and consider closer post spacing. Reducing panel spacing from 1.83m to 1.2m cuts the load on each post by a third.
Slatted or hit-and-miss panels that let wind through reduce the load substantially compared with solid close-board. On a genuinely exposed boundary that choice matters more than the post depth.
Digging the holes
A post hole auger is the tool for this, and hiring one for a day transforms a boring job. A petrol auger digs a 300mm hole to 600mm in under a minute in reasonable ground; a spade and a post hole digger take fifteen.
Augers struggle in stony ground and hit obvious trouble with tree roots and buried services. Check for services before you start — a cable detector costs little to hire and a severed supply cable costs a great deal more.
Keep the sides of the hole vertical rather than letting them flare out at the top. A bell-shaped hole uses more concrete and gives no extra grip, because the resistance comes from the soil bearing against the side of the collar rather than from the volume of concrete.
Setting the posts level
Run a string line between the two end posts of a run and set every intermediate post to it. Working post by post without a line accumulates error, and by the tenth post the fence visibly wanders.
Check plumb on two adjacent faces with a level, not one. A post that reads plumb on the face can still lean sideways, and it will be obvious once the panels go on.
For a fence running down a slope, decide early whether the panels will step down in level sections or follow the gradient. Stepped panels need posts of varying length; raked panels need the posts plumb but the panels cut to an angle.
Gate posts need more
A gate post carries a permanent asymmetric load — the weight of the gate hanging off one side, plus the shock of the gate being swung and slammed.
Increase depth by at least 150mm over the fence posts, use a bigger post section, and widen the hole. For a driveway gate, 900mm to 1m deep in a 450mm hole is a reasonable minimum, and heavy gates warrant a structural design.
Frost and ground conditions
In the UK, 600mm clears normal frost depth comfortably. In the US, posts should reach below the local frost line, which in northern states can be 1.2m or more — check the local figure rather than assuming.
Ground type changes things too. Sandy, free-draining soil gives good bearing but drains fast, so the concrete collar grips well. Soft clay gives poor bearing and may need a wider hole. Waterlogged ground needs a gravel base under the concrete so water can drain rather than sitting around the post.
The gravel base
Put 50 to 100mm of compacted gravel in the bottom of every hole before the post goes in. It costs almost nothing and does two useful things: it gives water somewhere to drain rather than pooling at the post base, and it stops the post sitting in a concrete cup that holds moisture permanently.
This single detail adds years to the life of a timber post, and it’s skipped far more often than it’s done.
Common questions
How deep should a 6ft fence post be?
600mm in a 300mm wide hole, which means a 2.4m post. Add 100mm on exposed sites, and more for gate posts.
Is it better to dig deeper or wider?
Wider, generally. Posts fail by rotating rather than sinking, and a wider concrete collar resists rotation far more effectively than extra depth does.
How much concrete for one fence post?
About 0.036m³ for a standard 300mm hole at 600mm deep with a 100mm post — roughly four 20 kg bags of postcrete. Round up per hole rather than across the whole job.
Can I set fence posts in the ground without concrete?
Compacted gravel works and drains better, which extends post life. It gives less rigidity than concrete, so it suits lighter fences on sheltered sites rather than tall close-board panels.
How far apart should fence posts be?
1.83m centres for standard UK panels. Closer spacing on exposed sites reduces the wind load carried by each post, which is often more effective than making individual posts deeper.