How deep should fence posts be? As a general rule, put at least one third of the post’s total length into the ground. For a standard 6ft fence, this usually means using an 8ft post and digging a hole around 2ft deep. Taller fences, exposed sites, heavy gates and loose ground may need deeper holes or larger posts.

Depth is only part of the job. The width of the hole, ground conditions, post type and amount of post mix also affect how securely your fence stands. Before digging, check for underground pipes, cables and drains, and confirm the boundary line.

Fence post depth rule of thumb

The one-third rule is a useful starting point: around one third of the post below ground and two thirds above ground. Common examples include:

3ft above ground: use a post around 4ft 6in long, with roughly 18in in the ground.
4ft above ground: use a post around 6ft long, with roughly 2ft in the ground.
5ft above ground: use a post around 7ft long, with roughly 2ft in the ground.
6ft above ground: use an 8ft post, with roughly 2ft in the ground.
7ft above ground: consider a 10ft post, with up to 3ft below ground, subject to site conditions.

These are practical guides rather than fixed engineering figures. A solid 6ft fence in a sheltered garden may perform well with 2ft-deep holes, while the same fence on an exposed corner could need additional depth, a wider footing or professional structural advice.

How wide should a fence post hole be?

A hole should normally be around three times the width of the post. For many domestic fence posts, that means a hole roughly 9in to 12in wide. Wider holes may be needed in soft, disturbed or wet ground.

A hole that is deep but barely wider than the post leaves too little room for concrete. Aim for a continuous collar of post mix around the post, not just a thin layer on one side. Keep the post centred and check it with a spirit level on two adjacent faces before the mix sets.

Try to make the sides reasonably straight and the base firm. If loose soil falls back into the hole, remove it before placing the post. Concrete poured onto loose spoil will not provide the same support as concrete bearing against undisturbed ground.

Depths for concrete and timber fence posts

The basic depth rule applies to both timber and concrete posts, but their handling and long-term performance differ.

Concrete fence posts

Concrete posts are heavy and need careful lifting, particularly longer sizes. Make the hole large enough to position the post without forcing it against the edge. Brace it securely while the post mix cures. Concrete posts work well with slotted fence panels and concrete gravel boards, creating a durable system that keeps timber panels away from damp soil.

Timber fence posts

Use pressure-treated timber intended for outdoor ground contact. Do not shorten a treated post without protecting the cut end with a suitable end-grain preservative. Shape the concrete surface so water runs away from the post rather than collecting around it. Persistent moisture at ground level is one of the main causes of early timber decay.

Do not rely on post depth alone to support a large gate. Gate posts carry repeated sideways and twisting forces, so they often need to be thicker, deeper and set in larger concrete footings than ordinary line posts.

How much post mix do you need?

The quantity depends on the hole’s diameter and depth, the post dimensions and the pack size. A typical domestic post hole may use roughly two bags of ready-to-use post mix, but narrow holes can use less and larger or deeper holes can require three or more. Treat any bags-per-hole figure as an estimate rather than a guarantee.

To plan your order, work out the approximate volume of each hole, subtract the volume occupied by the post, then allow extra for uneven sides, spillage and changes in ground level. Manufacturer coverage information on the bag should be your final guide.

It is sensible to have spare mix available. Running out halfway through a post can leave a weak joint between separate batches. K C Fencing and Timber Ltd supplies post mix and other building materials for trade and DIY fencing projects.

Using rapid-setting post mix

Follow the instructions printed on the product you buy, as water quantities and methods vary. In general, position and brace the post first, add the mix as directed, then add the specified amount of clean water. Check the post again immediately because rapid-setting products allow little adjustment time.

Do not hang heavy panels or gates straight away. Setting time and full strength are not the same thing, so follow the manufacturer’s guidance before loading the post.

Setting fence posts in clay soil

Clay is common across the West Midlands and needs extra thought. It can be hard when dry, sticky when wet and prone to holding water around the base of a post. Some clay also expands and contracts as moisture levels change.

Dig to firm, undisturbed ground: remove soft surface soil and loose material from the hole.
Avoid water-filled holes: if water keeps entering, find the cause and allow conditions to improve rather than trapping excessive water in the footing.
Do not create a water pocket: finish the top of the concrete slightly above ground level and slope it away from timber posts.
Allow for movement: a wider, properly filled footing can provide better resistance where clay shrinks, swells or becomes soft.
Take care near trees: roots make digging difficult, while nearby trees can affect moisture levels in clay.

A small amount of granular material at the base may help provide a level working surface, but it does not replace correct depth or a sound concrete footing. If the site is steep, waterlogged, close to a retaining wall or affected by significant ground movement, seek suitable professional advice before proceeding.

Adjusting depth for wind and fence design

Solid panels catch more wind than open styles. If your garden is exposed, do not assume a standard 2ft hole will suit every fence. Consider deeper or wider footings, stronger posts and a fence design that allows some wind through.

Post spacing matters too. Most standard fence panels are designed for posts at set centres, but always measure the actual panels, posts and gravel boards before digging. Mark every hole along a tight string line and check the overall run. You can also use the fence calculator to estimate the main materials required.

Sloping gardens may need stepped panels and posts of different lengths. Work from the finished top line and ensure each post still has enough material below ground. Do not simply raise a short post in its hole to match the next panel.

Common mistakes to avoid

Using posts that are too short for the intended fence height.
Digging shallow holes because the ground becomes difficult.
Making holes too narrow for enough post mix around the post.
Fitting panels before the footing has gained sufficient strength.
Failing to brace posts while the mix sets.
Letting concrete form a dip that holds rainwater against timber.
Using ordinary line posts for heavy or wide gates.
Ignoring underground services before digging.

Measure twice and buy the correct post length before starting. A post that is set too shallow is difficult to correct once panels are fitted, especially in clay or on an exposed site.

Ask K C Fencing and Timber Ltd

K C Fencing and Timber Ltd manufactures and supplies fencing materials for trade and DIY customers across the West Midlands. We deliver across the region, including Wolverhampton, Birmingham, Walsall, Dudley, West Bromwich, Worcester and surrounding towns, with free collection from our Wednesbury yard. We supply materials only and do not install fencing. Call 01902 288162 or request a quote for posts, panels, gravel boards and post mix.

How Deep Should Fence Posts Be Set?