Many fences that lean after the first spell of wind usually have the same weak point - the posts were too small for the job. If you are asking what size fence post is needed for your project, the right answer depends on more than just fence height. The type of fence, post spacing, ground conditions, and whether you are using timber or concrete can all make a difference.
Choosing the correct post size at the start saves time, money and a lot of frustration later. A fence post is doing structural work, not just separating panels. Get the posts right, and the rest of the fence has a far better chance of staying straight, secure and serviceable for years.
What size fence post you need depends on
Taller, heavier fencing requires larger, longer posts. That said, there is no single post size that suits every garden or boundary. A 6ft decorative panel in a sheltered suburban garden is very different from a closeboard run on an exposed ridge on the North Downs.
When deciding what size fence post is needed, start with four practical factors. The first is fence height above ground. The second is the weight and wind resistance the post has to support. The third is the spacing between posts. The fourth is the ground itself, because posts in soft or unstable ground provide less support than those in firm, well-compacted soil.
If any of those factors become more demanding, the post size usually needs to be adjusted accordingly.
Start with the height of the fence.
For domestic fencing, the post should be long enough to allow a minimum of 600mm in the ground. Depending on fence height and site conditions. As a general guide, between a quarter and one third of the overall post length should be in the ground. That is not an absolute rule, but it is a reliable starting point.
For a 1.8m fence, which is one of the most common garden fence heights in the UK, posts are often around 2.4m long. That allows for the minimum 600mm in the ground. In more exposed areas, or where the fence is made up of longer bays between posts, going deeper is sensible.
A lower fence, or one with a trellis top, still needs the correct-length post, as it can still catch the wind. Trellis-topped fencing is a good example. Even though the structure may look lighter, trellis still has a lot of wind resistance and creates extra strain on the post.
Common timber fence post sizes
Timber posts are popular because they are easier to handle and cut on site, and they suit the look of a traditional garden fence. The key is choosing a section size that matches the load.
For lighter domestic fencing, 75mm x 75mm timber posts are sometimes used, but these are generally best kept for lower fences, lighter screening or short runs in sheltered gardens. They can be adequate in the right setting, but they do not leave much margin for movement or long-term wear. If you particularly want a 75mm face width, use a 75mm x 100mm post, as the 100mm depth makes the post much stronger.
For many standard garden fence projects, 100mm x 100mm posts are a more dependable choice. They offer noticeably better strength and are commonly used with 1.8m fence panels, featheredge fencing and general boundary work.
Where fencing is taller, heavier or more exposed, 125mm x 100mm timber posts can be the better option. They cost more and are heavier to work with, but they give extra rigidity and can be worthwhile where strength matters.
The quality of the timber's treatment matters too. A correctly sized post can still fail early if it has not been pressure-treated for ground contact or if water is allowed to sit around its base.
Concrete posts and DuraPost change the calculation
Concrete fence posts are often chosen for their durability and lower long-term maintenance needs, as they will not rot in the ground. Concrete slotted posts suit most panel styles well, and concrete mortised posts are good for closeboard fencing, particularly around domestic boundaries.
Because concrete is inherently stronger and more resistant to decay than timber, the section sizes do not compare directly on a like-for-like basis. Instead, the important point is choosing a concrete post designed for the fence height and type.
Whereas for Durapost, there are only two types to choose from: The Classic range and the heavier Commercial range, so the important part is getting the post length right.
Concrete posts can be an excellent option where customers want a longer-lasting structure with less upkeep. The trade-off is weight and looks. They are harder to move, installation can be slower, and the look may not suit every setting. Beware! Many professional fencers have bad backs because of concrete posts.
Like Concrete posts, Durapost does not suffer from rot issues and has an excellent lifespan. The real plus point for DuraPost is how light it is. 50% or more lighter than the equivalent height concrete post.
Fence type matters as much as fence height
A lightweight decorative panel and a closeboard bay of the same height do not place the same demand on the posts. Closeboard fencing, hit-and-miss fencing, and acoustic panels all use longer bays and all create more load on individual posts than simple overlap panels.
That is why asking only for a post to suit a 6ft fence can lead to the wrong answer. A 6ft run of heavy closeboard in an exposed position will need more substantial posts than a run of 6ft lap panels in a sheltered back garden.
Gates are another area where undersized posts cause problems. Gate posts need to cope with constant movement, hinge strain and supporting the weight of the gate itself. They often need to be larger than the line posts used along the rest of the fence.
Ground conditions can change everything
Soil type directly affects post performance. Firm ground with good drainage gives a much better base than soft, sandy or waterlogged ground. If the ground is loose or holds water, even a correctly sized post can shift over time if it is not installed deep enough or set properly.
Clay soils can also be awkward because they expand and contract with changing moisture levels. In these areas, a longer post and careful installation can help reduce movement. There are several differing ideas on how to set posts into clay soil. If you have heavy clay soil, have a look on Google for the latest advice on setting posts in clay soil. It is also worth checking with local fencers to see what works in your area.
If you know the site is exposed to strong winds, it is usually wise to size up rather than work to the bare minimum.
A practical guide to what size fence post is needed
For a modest domestic fence up to around 1.2m high, a 75mm x 75mm timber post may be suitable in some sheltered situations, though many customers still prefer 100mm x 100mm for extra reliability.
For standard 1.8m garden fencing, 100mm x 100mm timber posts are a common and sensible choice. If the fence is especially heavy and uses long bays, or the site is exposed, 125mm x 100mm posts are often worth considering.
Concrete mortised posts are roughly 100mm x 100 mm for all heights. Concrete slotted posts are a little smaller and slightly tapered.
DuraPost Classic is a uniform, strong "H" section all the way up, and can be used for panels, closeboard, picket, and most types of domestic fencing.
For wider gates, larger timber posts are usually a safer option, with the size chosen being based on gate width, weight, height, and use. If you are not sure about the gate post size, we can help.
Do not overlook post spacing and installation depth
Panel systems are usually designed around set bay widths of 1.8m for continental-style panels and 6ft (1.83m) for UK panels.
Closeboard supplied in kit form utilises longer 2.4m or 3m bays. It is not a good idea to use the longer 3.6m bays; these should be reserved for when there is no other option.
Installation depth is just as important. A shallow post hole is one of the most common reasons a fence fails early. As a guide, posts for typical garden fencing should be at least 600mm into the ground, with deeper settings used for taller fencing, longer bays, softer ground or exposed sites.
Concrete around the base should support the post without trapping water. The top of any concrete around posts should slope away from the post so that a puddle does not form around the base of the post when it rains. For timber posts, drainage and suitable post treatment are part of the job, not an extra.
Timber, Concrete or DuraPost- which is better?
There is no universal winner. Timber posts can look more natural and are often easier for DIY work. They are a good fit for many garden projects, especially where appearance matters and installation access is tight.
Concrete posts offer durability and consistent strength, which is why many homeowners and contractors choose them for long-term boundary fencing. They can be particularly useful where replacing rotten timber posts every few years is not appealing.
DuraPost are light and easy to handle, strong and durable, as well as easily adaptable for all types of domestic fencing. DuraPost has several major advantages over Concrete posts, they do not chip or crack and are much less obtrusive to look at.
The better choice depends on budget, appearance, handling, fence style and how long you want the structure to last with minimal maintenance.
When is it worth asking for advice
The above are practical starting points, not fixed rules. If the fence has unusual loading, awkward ground, or a combination of trellis and solid panels, a more cautious approach is usually best.
If your project includes retaining changes in level, heavy gates, agricultural fencing, equestrian areas or exposed coastal and rural conditions, standard assumptions may not be enough. This is where specialist advice can save wasted materials and repeat work.
For DIY and Trade customers across Kent, East Sussex and Surrey, Crestala Fencing Centre often helps with exactly these practical questions - matching posts to panels, gates, ground conditions and the realities of the site rather than giving a one-size-fits-all answer.
If you are unsure, take the fence height, style, total run length and a rough idea of ground conditions with you when you ask. That makes it much easier to choose the right post first time.
A fence only performs as well as the posts holding it up. Give them enough size, enough depth and the right material for the setting, and the whole job becomes more straightforward from there.
