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How Deep Should a Fence Post Go? A Third of Its Height

A third of the post above ground, buried, is the working rule — and below the frost line wherever ground freezes. Width matters as much as depth.

By Zakaria 3 min read Save to Pinterest — opens Pinterest in a new tab
A line of square timber posts set into mown grass, receding into the distance
Photo: PublicDomainPictures, Pixabay Content License, via Pixabay
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A third of the post’s above-ground height, buried — so a 6 foot fence wants about 2 feet in the ground. Two things override that rule: the frost line, and soft ground. And hole width matters as much as depth, because fences fail sideways.

Depth by fence height

This table answers the question on its own
Fence heightBuried depthPost length
4 ft~16 in~5 ft 4 in
6 ft~2 ft~8 ft
8 ft~2 ft 8 in~10 ft 8 in
Gate posts, any heightDeeper and widerAdd material

Frost line beats the third rule

Water in soil expands as it freezes and lifts anything whose base sits in the freezing zone — unevenly, so posts heave and lean over successive winters. Local frost depth varies enormously; the building department or any local fencing contractor will know the figure.

Width, and why fences fail sideways

A fence panel is a sail. Wind pushes at the top of the post and levers the base through the soil.

Resisting that depends on how much material surrounds the post, which is why a hole about three times the post width — around 12 inches for a 4x4 — performs so much better than a narrow deep one. Depth and width work together; depth alone does not.

Concrete or gravel

This table answers the question on its own
ConcreteCompacted gravel
RigidityHighestGood
Drainage at the collarCan hold waterFree-draining
Replacing the post laterHardEasy
Best forGate posts, exposed runsMost ordinary fencing

The usual compromise: a gravel base beneath a concrete collar, so water running down the post has somewhere to go instead of sitting against the timber.

Dome the concrete, and slope it away

Finish slightly above ground level, sloping away from the post. A flat or dished finish holds a ring of water at the collar — which is exactly where posts rot while the rest of the timber is still sound.

Why posts lean

  1. Too shallow — the base walks a little further with each gale
  2. Too narrow a hole — not enough material resisting the leverage
  3. Frost heave — base above the frost line
  4. Rot at the collar — which finishes what the movement started

All four are set on the day the post goes in, which is the argument for doing it properly once.

Before you dig

  • Check for buried services. Utilities, drains, irrigation, cables to a shed.
  • Check the boundary line, not the line of the old fence — they are not always the same.
  • Set the end and corner posts first, then run a string line and work along it.

Common questions

How deep should a fence post go?

A working rule is a third of the post's above-ground height buried in the ground, so a 6 foot fence wants about 2 feet below ground and a post around 8 feet long. Two conditions override that. Wherever the ground freezes, the hole must go below the local frost line regardless of what the third rule suggests, or frost heave will lift the post over successive winters. And on soft or sandy ground, going deeper and wider is what provides the resistance that firm soil would have given.

Does the frost line matter more than the depth rule?

Yes, where ground freezes it is the governing requirement. Water in soil expands as it freezes and lifts anything whose base sits within the freezing zone, and because it does not lift evenly the post ends up progressively heaved and leaning. Setting the base below the frost line puts it in ground that stays unfrozen. Local frost depth varies enormously, and the building department or any local fencing contractor will know the figure.

How wide should a fence post hole be?

About three times the post width is the usual guidance, so a 4x4 post wants a hole around 12 inches across. Width matters as much as depth because a fence fails by being pushed over rather than pushed down, and resisting that overturning force depends on the mass and spread of what surrounds the post. A narrow deep hole is less effective than a properly wide one at the same depth.

Should fence posts be set in concrete or gravel?

Both work and they fail differently. Concrete gives the most rigid result and is the usual choice for gate posts and exposed runs, but it can hold water against the timber at the collar, which is exactly where posts rot. Compacted gravel drains freely, lets a post be replaced easily and performs well in most soils, at some cost in rigidity. A frequent compromise is a gravel base beneath a concrete collar, so water draining down the post has somewhere to go.

Why do fence posts lean over time?

Usually because they were not deep enough, not wide enough, or were set in ground that holds water. Wind loads a fence panel like a sail and pushes at the top of the post, which levers the base through the soil, so a shallow post walks a little further each gale. Frost heave does the same job vertically where the base sits above the frost line. Rot at the collar then finishes what the movement started.

Should concrete be domed at the top of a post hole?

Yes, and it is a detail worth getting right. Finishing the concrete slightly above ground level and sloping it away from the post sheds water off rather than letting it pool against the timber at the collar. A flat or dished finish does the opposite and holds a ring of water around the post, which is why so many posts rot precisely at ground level while the rest of the timber is sound.

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