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2026-09-11
Choosing the right post base for concrete starts with one simple detail: the actual size of your wood post. A nominal 4×4 post, for example, usually measures about 3.5 inches square. The same difference applies to larger posts such as 6×6, 8×8, 10×10, and 12×12.
That matters when you are buying a post base. If the opening is too small, the post will not fit. If the dimensions do not match, installation can become difficult and the connection may not work as intended.
This guide explains how post base sizes work, what to measure before buying, and how to choose the right option for a pergola, deck, patio, gazebo, or similar outdoor project.
A post base connects a wood post to a concrete surface. Instead of placing the bottom of the post directly against the concrete, the base creates a defined connection between the two materials.
Many post bases also raise the wood slightly above the mounting surface. This gap is commonly called a standoff.
The idea is simple. Keep the bottom of the wood away from direct contact with the concrete and provide a solid mounting point for the post.
For outdoor projects, this can be useful around areas where moisture, rain, or occasional standing water may occur.
A concrete post base can work with many types of projects, including:
The correct product depends mainly on the size of the post and the type of surface you are mounting it to.
This is where many first-time buyers get confused.
A wood post labeled 4×4 does not usually measure exactly 4 inches by 4 inches. A standard finished 4×4 post often measures approximately 3.5 inches by 3.5 inches.
The same pattern applies to larger nominal sizes.
| Nominal Wood Post | Approx. Actual Size |
|---|---|
| 4×4 | 3.5″ x 3.5″ |
| 6×6 | 5.5″ x 5.5″ |
| 8×8 | 7.5″ x 7.5″ |
| 10×10 | 9.5″ x 9.5″ |
| 12×12 | 11.5″ x 11.5″ |
So, don’t choose a base based only on the number printed on the lumber.
Measure the actual post first.
That one step can save you from ordering the wrong size.
The easiest way to choose a post base is to match the base with the actual dimensions of the wood post.
For example, if your nominal 4×4 post measures 3.5″ x 3.5″, look for a 4×4 post base that matches that dimension.
The same approach works for larger posts.
A nominal 4×4 post usually has an actual size of about 3.5″ x 3.5″.
A 4×4 post base for concrete is a common choice for smaller outdoor structures and suitable deck, patio, porch, and pergola projects.
Before ordering, measure your post. Older lumber, custom-cut posts, and specialty materials may have different dimensions.
If you are working with a standard 4×4 post, you can see the EONECROSS 4×4 post base for concrete for its actual fit and installation details.
A nominal 6×6 post usually measures around 5.5″ x 5.5″.
A 6×6 option gives you a larger connection point for projects that use larger wood posts.
For a standard 6×6, check that your post measures approximately 5.5″ square before installation.
If your project uses this size, take a look at the EONECROSS 6×6 post base for concrete.
An 8×8 post typically measures about 7.5″ x 7.5″.
At this size, accurate measurement becomes even more important. A half-inch difference can make a noticeable difference when fitting the post into the base.
For a nominal 8×8 post with an actual size of 7.5″ x 7.5″, an 8×8 post base can provide the appropriate size range.
A nominal 10×10 post usually has an actual size of approximately 9.5″ x 9.5″.
Large wood posts often appear in pergolas and other substantial outdoor structures. Before buying a 10×10 base, measure the post at several points if the lumber is not perfectly uniform.
For a standard 9.5″ x 9.5″ post, see the EONECROSS 10×10 post base for concrete.
A nominal 12×12 post generally measures about 11.5″ x 11.5″.
This is the largest size in the current EONECROSS post base range.
If your post measures approximately 11.5″ square, a 12×12 post base for concrete may be the appropriate size.
As always, check the actual dimensions before installation.
If you have ever bought dimensional lumber, you have probably noticed that the advertised size does not always match the tape measure.
A 4×4 post is a good example.
The name refers to the nominal lumber size. After milling and finishing, the actual dimensions are usually smaller.
That gives us:
Nominal size: 4×4
Actual size: approximately 3.5″ x 3.5″
The same principle applies to 6×6, 8×8, 10×10, and 12×12 posts.
This is why product listings should provide both the nominal post size and the actual opening.
Do not assume that every piece of lumber has identical dimensions.
For a better fit:
This is especially important when using reclaimed wood, rough-sawn lumber, or custom-cut posts.
A standoff is the raised section that keeps the bottom of the wood post above the concrete or mounting surface.
For example, EONECROSS post bases in this product range use a 1-inch standoff.
The purpose is straightforward. The wood does not sit directly on the concrete.
This can help limit direct exposure to moisture around the bottom of the post. It also gives the connection a little separation from the mounting surface.
When comparing post bases, check the standoff height along with the post opening.
You may see terms such as:
These terms can describe similar product categories, but the actual design and installation method can vary.
Pergolas commonly use 4×4, 6×6, or larger wood posts, depending on the design.
There is no single post base size that works for every pergola.
Instead, start with the lumber.
For example:
For a pergola project, you may also need pergola brackets to connect the wood framing.
A post base is one part of the overall connection. It does not replace proper structural design.
One reason people choose a post base is to create separation between the wood and concrete.
With direct wood-to-concrete contact, the bottom of the post remains against the mounting surface.
A raised post base creates a different connection.
The right method depends on the project, local requirements, and the specific materials involved.
Installation varies by product, so always follow the instructions supplied with the base and use compatible anchors and fasteners.
A typical installation looks like this:
Measure the actual wood post before positioning the base.
Make sure the dimensions match the product specifications.
Place the base where you want the post to sit.
Check the orientation and make sure the base sits properly on the concrete.
Use the holes in the base to mark the locations for the concrete anchors.
Take your time here. Accurate positioning makes the next steps easier.
Drill the mounting holes according to the anchor manufacturer’s requirements and the product instructions.
Remove dust and debris from the holes before installing the anchors.
Install the appropriate expansion anchors or other specified fasteners.
Make sure the hardware fits both the base and the concrete.
Tighten the mounting hardware and check that the base remains properly aligned.
Place the post into the base and secure it using the specified fasteners.
Finally, check the post for alignment before continuing with the rest of the structure.
Size is the first thing to check, but it is not the only one.
Look for the actual opening dimensions rather than relying only on the nominal lumber size.
A thicker steel construction can provide a more substantial bracket for suitable outdoor applications.
For example, EONECROSS post bases use 2mm steel across this product range.
Check how far the base raises the post above the concrete.
EONECROSS bases in the 4×4 through 12×12 range use a 1-inch standoff.
Outdoor hardware needs a finish suited to the environment.
EONECROSS post bases are available with different surface finishes depending on the model. Black models use an e-coated finish, while silver models use galvanized steel. Both finishes help protect the steel from normal outdoor exposure.
When choosing between them, you can mainly focus on the post size, installation requirements, and the finish you prefer for your project.
Check what comes with the package.
The EONECROSS post base products include expansion anchors and self-tapping screws. However, the exact hardware requirements can depend on the mounting surface and project.
Here’s a quick way to narrow it down.
Have a standard 4×4 post?
Start with a 4×4 post base.
Using a 6×6 post?
Look at a 6×6 post base.
Working with an 8×8 post?
Check the actual 7.5″ x 7.5″ dimensions and choose an 8×8 base.
Using a 10×10 post?
Look for a base designed for approximately 9.5″ x 9.5″ actual lumber.
Have a 12×12 post?
Check for an approximately 11.5″ x 11.5″ opening.
If your post doesn’t match these measurements, don’t force the fit. Find a base with dimensions that match your actual post.
A few simple mistakes can cause problems later.
Seeing “6×6” on the lumber does not mean the post actually measures 6 inches square.
Always check the actual dimensions.
The height of the base can affect how the finished post sits.
Check the standoff before ordering.
Not every concrete installation is the same.
Consider the condition and thickness of the concrete and make sure the selected anchors are suitable.
A strong bracket still needs the right hardware.
Follow the product instructions and anchor manufacturer’s specifications.
Outdoor structures can be subject to local building codes.
For a pergola, deck, or other structural project, check the requirements in your area before installation.
EONECROSS offers several sizes for common nominal wood post dimensions.
| EONECROSS Size | Actual Post Fit | Standoff |
|---|---|---|
| 4×4 | 3.5″ x 3.5″ | 1″ |
| 6×6 | 5.5″ x 5.5″ | 1″ |
| 8×8 | 7.5″ x 7.5″ | 1″ |
| 10×10 | 9.5″ x 9.5″ | 1.2″ |
| 12×12 | 11.5″ x 11.5″ | 1″ |
All models use 2mm steel construction and are intended for suitable concrete-mounted outdoor applications.
Before ordering, compare your actual post dimensions with the product specifications.
A: A standard nominal 4×4 wood post usually measures approximately 3.5″ x 3.5″. A 4×4 post base is designed around this common actual dimension. Always measure your post before ordering.
A: A nominal 6×6 post commonly measures about 5.5″ x 5.5″. Check the actual post dimensions and compare them with the base specifications.
A: Yes, post bases are commonly used to create a connection between wood posts and concrete. The specific anchor and installation method should match the product and concrete conditions.
A: A 1-inch standoff raises the bottom of the wood post approximately 1 inch above the mounting surface. This creates separation between the wood and concrete.
A: Usually not. A standard finished 4×4 post typically measures about 3.5″ x 3.5″. Nominal lumber dimensions and actual dimensions are different.
A: They can be used for suitable pergola projects when the post size, concrete surface, fasteners, and overall structure are compatible. Always follow local building requirements.
A: Generally, yes. The base should match the actual dimensions of the wood post. For example, a nominal 8×8 post typically uses a larger base than a nominal 4×4 post.
A: Yes. Measuring the actual post is one of the easiest ways to avoid ordering the wrong size.
The best post base for concrete is not necessarily the largest or heaviest option. It is the one that matches your actual post dimensions and works with your concrete surface and installation requirements.
For standard lumber, the basic size guide is easy to remember:
4×4 → 3.5″ x 3.5″
6×6 → 5.5″ x 5.5″
8×8 → 7.5″ x 7.5″
10×10 → 9.5″ x 9.5″
12×12 → 11.5″ x 11.5″
Measure first, check the product specifications, and choose the base that fits your post.
For pergolas, decks, patios, and other outdoor projects, that simple approach can make the buying and installation process much easier.
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