The post hole is not the place to save time. If a fence is going to fail, it usually starts underground.

Most leaning fences come back to one of three things: the post was not deep enough, the post was not suitable for ground contact, or there was not enough postmix around it. The fence may look fine on day one, but once the wind gets behind the panels, rails or boards, the post starts working like a lever.

That is why the real question is not just “how deep should fence posts be?” It is “what post length do I need, how deep should I dig, and what will stop the fence moving later?”

The simple fence post depth rule

For normal garden fencing, start with this:

• Minimum depth: 600mm
• 6ft fence: usually 600mm–750mm
• Exposed runs: consider 750mm–900mm
• Soft or wet ground: go deeper if needed
• Gate posts: treat separately

The common “one-third rule” is useful, but it needs saying properly. For fencing, it normally means allowing roughly one-third of the above-ground fence height below ground. So, if you want around 6ft above ground, the usual starting point is around 2ft below ground.

That is why a 6ft finished fence is not a 6ft post job. You need enough extra post length to bury.

What length fence post do I need?

This is where many fencing jobs go wrong before the first hole is dug. Customers plan the visible fence height and forget the part of the post that disappears into the ground.

This table is a practical buying guide, not a structural design calculation. The post still needs to suit the fence type, ground conditions, exposure and fixing method.

Fence post depth guide

Use this as a practical starting point, then adjust for the job in front of you.

Not every post in a fence run does the same job

A straight run of fencing may look simple, but different posts take different forces. Treating every post exactly the same is another common mistake.

• Intermediate posts carry the normal run of panels, rails or boards.
• End posts take more stress because the fence stops there.
• Corner posts take load from two directions.
• Gate posts carry moving weight from the gate and hinges.
• Agricultural strainer posts need to handle tension from wire and stock fencing.

For a normal domestic panel fence, the intermediate posts are usually straightforward. Corners, ends and gates need more care. If the job includes a heavy gate, exposed corner, slope or strained wire, do not base the post depth on a standard garden fence panel.

What changes how deep a fence post should be?

There is no honest single answer for every fence. The right depth depends on what the post has to resist.

Fence height

The taller the fence, the more wind it catches. A low picket fence is not putting the same pressure on the post as a 6ft closeboard or featheredge fence.

Ground conditions

Soft, wet, made-up or recently disturbed ground gives less support than firm, stable ground. If the hole sides crumble, hold water, or feel unstable, a shallow post will not improve with time. It will usually get worse after wind and rain.

Wind exposure

Open gardens, hillier areas, fields, corner plots and coastal locations can all put more force through a fence. In exposed positions, the post depth, post size and fixing method all need upgrading together.

Post size and fence weight

Heavier fencing needs stronger posts and a better footing. A narrow post set shallow may look fine on day one, but it has very little resistance once the fence starts moving.

Do fence posts need concrete or postmix?

For most permanent domestic fencing, yes. Postmix or concrete helps lock the post in position and stops the surrounding ground from loosening as quickly.

Postmix is not a magic fix for a shallow hole. If the post is not deep enough, adding more mix at the top will not make it behave like a properly set post. Depth first, then fixing.

• Dig the hole deep enough
• Keep the post upright and aligned
• Use enough postmix for the hole size
• Slope the top away from the post
• Let the fixing cure before loading the fence

How much postmix do you need per fence post?

As a practical guide, many standard fence posts need around 2 bags of postmix per hole. Larger holes, deeper holes, exposed locations and gate posts may need more. The old blog used 2 to 3 bags as a common site guide, which is still sensible as long as it is not treated as a guarantee.

The mistake is trying to save a bag and leaving too much loose backfill around the post. Soil alone does not grip the post like a properly filled and compacted fixing.

If you are planning a full featheredge run, the Wern-Wood Featheredge Fencing Calculator can help you work out posts, rails, boards, gravel boards and postmix before ordering.

Use the right post for ground contact

Depth is only one part of the job. The post also needs to be suitable for where it is being used. Timber that is going into the ground should be suitable for ground contact. In treatment terms, that normally means Use Class 4.

This is where customers can get caught out. “Treated” does not automatically mean “right for every outdoor job”. Fence boards, rails and above-ground treated timber do a different job to a post buried in wet ground.

Check before you dig

Fence post holes are small excavations, but they are still holes in the ground. Before digging, think about underground services, especially near houses, drives, paths, outbuildings and boundaries.

Electric cables, water pipes, drainage, lighting supplies and other services may be shallower or less obvious than expected. If you do not know what is below, do not just drive a bar or auger into the ground and hope for the best.

• Check plans or known service routes where available.
• Look for inspection covers, outside taps, ducting, lighting, sheds and previous groundworks.
• Take extra care near buildings, drives, walls and existing services.
• Use safe digging practice if there is any chance of buried services.

Common mistakes when setting fence posts

• Using a 6ft post for a 6ft fence — you still need enough post below ground.
• Digging too shallow — the biggest cause of leaning posts.
• Using the wrong post — above-ground treated timber is not the same as ground-contact timber.
• Using too little postmix — loose backfill lets the post move.
• Ignoring wet or soft ground — poor ground needs a better footing.
• Treating gate posts like normal posts — gates add moving weight and hinge pressure.
• Loading the post too soon — let the fixing cure before putting the fence under strain.
• Forgetting wind exposure — exposed fences need more support than sheltered ones.

Before you order fence posts

Before ordering, work out the full job, not just the visible fence height.

• Finished fence height — this tells you the visible height you want.
• Buried depth — this tells you how much post length disappears underground.
• Post length — visible height plus buried depth.
• Post type — intermediate, end, corner, gate or agricultural post.
• Ground conditions — firm, soft, wet, sloping or made-up ground.
• Postmix quantity — allow enough for the hole size and post type.
• Fixings and rails — make sure the rest of the fence system matches the posts.

When you are ready to order, browse Fence Posts and add the right Postmix & Fence Fixings at the same time. That way, you are not halfway through digging before realising the fixing materials are missing.

FAQs

Is 600mm deep enough for fence posts?

For many standard domestic fence posts, 600mm is a sensible minimum depth. It may not be enough for taller fencing, soft ground, exposed locations, heavy panels or gate posts.

How deep should posts be for a 6ft fence?

For a 6ft fence, posts are usually set around 600mm to 750mm deep. In exposed areas, wet ground or heavier-duty fencing, going closer to 750mm or beyond may be the better option.

What length post do I need for a 6ft fence?

For a standard 6ft finished fence, an 8ft post is the usual starting point because it allows around 6ft above ground and 2ft below ground. Exposed sites or heavier-duty work may need a longer or stronger post.

Should one-third of the post be underground?

The more useful fencing rule is to allow roughly one-third of the above-ground fence height below ground. For example, a 6ft fence commonly starts with about 2ft of post in the ground.

Can fence posts be too deep?

In normal fencing work, extra depth usually improves stability. The real limits are practical: digging effort, post length, ground conditions, drainage, underground services and the amount of postmix needed.

Can I set fence posts without concrete?

For temporary or light-duty work, posts can sometimes be backfilled without concrete. For permanent garden fencing, postmix or concrete is usually the better choice because it helps lock the post in position and reduce movement.

Do I need special posts for the ground?

Yes. Timber posts going into the ground should be suitable for ground contact. Look for posts treated for that use, rather than assuming any treated timber is suitable below ground.

Summary: set the post for the job, not the guess

If you want the simple version, start at 600mm deep for normal garden fencing and increase the depth when the fence is taller, heavier, more exposed, or going into poor ground.

But do not stop at the hole depth. Work out the finished fence height, choose a post long enough to bury properly, use a ground-contact post where timber is going into soil, and allow enough postmix to secure it properly.

That is what keeps a fence standing after the first wet winter and the first proper wind.

Sources checked: HSE guidance on avoiding danger from underground services; Gov.Wales planning guidance for fences, gates and garden walls; Wood Protection Association and Timber Development UK guidance on Use Class 4 treated timber. This article is general guidance only. Always assess the site, ground conditions, fence design and any local planning restrictions before starting work.