Are you standing in a timber yard with one hand on an 88x88mm post and the other on a 90x90mm post, and have no idea which one your project actually needs?

If that sounds familiar, you are in good company.

Timber post sizes in Australia are described using a mix of width, depth, length, treatment and grade. It is easy to get lost somewhere between the number on the tag and the post that turns up on site.

This guide breaks down how timber post sizes are described, what actually affects the size you need, and how pine and Cypress options compare. So you can walk into your next fencing, pergola, gate or retaining wall project knowing exactly what you are ordering.

How are Timber Post Sizes Described?

Timber posts are described using width, depth and length. A post is listed as 90x90mm, which means 90mm wide and 90mm deep. But they come in different lengths, often measured in lineal metres (for example, 3.0 LM). Square timber posts with the same width and depth are the most common choice for fencing, pergolas, and outdoor structures. This is because they suit a wide range of fixings.

Beyond the physical dimensions, a post is also described by:

Treatment level (such as H4), relating to where the timber can be used.
Stress grade (such as F7), relating to structural performance.
Surface finish, dressed or rough-sawn.
Manufacturing type, solid or laminated.

These factors don’t change a post’s width and depth, but they affect performance and suitability.

Categories of Timber Posts Available At Timber Central

Rather than a single one-size-fits-all product, timber posts are generally grouped into a few broad categories based on species, treatment and finish. At Timber Central, the current post range covers:

Treated Pine Posts – softwood posts treated to a hazard level such as H4, available as finger-jointed laminated or solid DAR (dressed all round) posts for a straight, consistent, ready-to-install finish.
Cypress Posts – naturally durable softwood posts, typically supplied rough-sawn and structurally graded (such as F7), for a more natural look without chemical treatment.

Sizing, finish, and treatment can vary within each category, so check the specific listing before you order.

What Affects the Timber Post Size You Need?

There is no single size that suits every job. What you need depends on the following:

The application, such as fencing, a pergola or a retaining wall.
The species, like treatment or grading of the timber.
Any project-specific requirements, including engineering or Australian Standards requirements for your build.
Also, the finish you want, whether you are after a dressed look or something routered and more natural.

These things change from site to site, so it is best to check sizing against your own plans, or with a qualified professional, rather than assuming one size works for everything.

Posts for Fencing, Pergolas, Gates and Retaining Walls

Not every job calls for the same post. The way a post will be used often says more about the right pick than the size tag alone:

Fencing: Timber posts are one of the most common uses, with both pine and Cypress widely used. Size and finish often come down to fence style, fixings, and the look the owner wants.
Pergolas and Verandahs: Posts are on full display here, not hidden like fence posts, so straightness and finish matter just as much as size. That’s where laminated posts like the finger-jointed option in the range earn their keep, staying straighter over time.
Gates: The added weight and movement of a swinging gate means sizing needs to suit the hardware and opening width.
Retaining Walls: Posts sit under load from soil and moisture. So treatment level and grading are particularly relevant, and you should check structural loads against engineering requirements before choosing timber.

Treated Pine Posts vs Cypress Posts

Treated pine and Cypress are the two main options in the range, and customers often land on one or the other after weighing up a few key differences.

	Treated Pine Posts	Cypress Posts
Durability	Relies on chemical treatment (such as H4) for termite and decay resistance	Naturally durable, thanks to oils in the timber, supplied without chemical treatment.
Ground contact	Rates for in-ground use through its treatment level	Suitability depends on grading, installation and exposure
Finish available	Finger-jointed laminated or solid DAR (dressed)	Typically rough-sawn
Look	Smooth, consistent, easy to paint or stain.	Natural grain and colour variation

Cypress pine is technically a softwood, but it’s often compared to some hardwoods in structural performance thanks to its natural oils, which make it less attractive to termites than many other untreated softwoods, though it’s not termite-proof.

Picture two customers at the yard, both setting posts for a fence line. Because the posts sit in constant ground contact, that application falls under the H4 hazard class set out in AS/NZS 1604, so the first customer selects a dressed H4 treated-pine post to meet it. The second wants a rustic look and picks a rough-sawn F7 Cypress post, relying on natural durability rather than chemical treatment. Both end up with timber suited to the job; they’ve just chosen different species and finishes.

Are you also looking for the high-quality pine post? Contact our team or visit our warehouse to see the range in person.

What Does H4 Mean on a Timber Post?

H4 is a hazard treatment level applied to pine, suited to applications involving ground contact or higher moisture exposure, such as posts, sleepers and retaining walls. It describes treatment only, not size or strength grade.

What Does F7 Mean on a Timber Post?

F7 is a structural stress grade that relates to strength and load capacity rather than size or finish. A rough-sawn F7 post carries the same rating as a dressed F7 post of the same species and dimensions. Only the surface finish differs.

Solid, Laminated, Dressed and Rough-Sawn Timber Posts

Manufacturing type and surface finish are easy to confuse with strength, but they describe two different things entirely:

Type	What It Means
Solid	Cut from a single piece of timber, with no joints or lamination
Laminated	Built from smaller sections, such as finger-jointed pine, joined together, which can help a post stay straighter
Dressed (DAR)	Machined smooth on all four faces
Rough-sawn	Left with its natural sawn surface, not planed smooth.

None of these describes strength grade on their own. A solid post isn’t automatically stronger than a laminated one, and a dressed post isn’t automatically stronger than a rough-sawn one, when the grade and species match.

Finished Size vs Nominal Size, and Does a Larger Post Mean a Stronger Post?

Two quick things worth knowing before you order:

Some timber is sold under a nominal size. That is slightly different from its actual finished size once it is dressed or milled. This is why similar sizes, like 88x88mm and 90x90mm, exist side by side. Always check the exact dimensions on the product listing.
A bigger post isn’t automatically stronger. Strength comes down to species, grade, treatment and installation, not just width and depth. A smaller, well-graded post can suit a job as well as a larger one.

Common Timber Post Selection Mistakes

A few slip-ups come up again and again when customers compare timber post sizes:

Assuming a larger post is always stronger, without checking grade or treatment.
Confusing nominal and finished sizes, resulting in a post that doesn’t match a bracket.
Choosing an untreated or lower-treatment post for an in-ground application.
Assuming rough-sawn and dressed posts of the same grade perform differently, when only the finish differs.

Final Summary

Timber post sizes in Australia are more than a number on a tag. Width, depth, length, treatment level, stress grade and surface finish all play a part, and pine and Cypress options each bring a different mix of these factors. No single size or type suits every project, so match post size, treatment, and grade to your application, and check against project requirements where needed, to choose the right post rather than guess.

FAQs

What are common timber post sizes in Australia?

Common timber post size options are 88x88mm, 90x90mm and 100x100mm, though sizing varies by supplier.

What size timber post should I use?

This depends on the application, load requirements, treatment level and any regulatory requirements.

Is a 90x90mm post suitable for a pergola?

It can be. It depends on the pergola design and loads involved. So, confirm against your specific plans first.

What is the difference between an 88x88mm and a 90x90mm post?

The size differs slightly in width and depth, and may also differ in the manufacturing type of finish. So check the product listing.

What is the difference between H4 treated pine and F7 timber?

H4 refers to the level of preservative treatment in timber. On the other hand, F7 refers to the structural strength grade. H4 treated pine focuses on protection against outdoor exposure, whereas F7 indicates the timber’s strength rating.

Are Cypress posts termite-treated?

No. They are naturally less attractive to termites because of the timber’s natural oils. So, they might be termite-proof, but no other treatment is applied to the post.

Are dressed and rough-sawn posts the same size?

Not always. Dressing can remove a small amount of material. So, the two finishes may have slightly different dimensions at the same nominal size.

Can timber posts be installed directly in the ground?

Some suit in-ground use depending on treatment level or natural durability, while others don’t, so always check first.

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