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How To Measure Your Posts & Post Caps

Finding The Right Post Cap For You

Finding the right post cap for your deck or railing can be a difficult and time consuming process. With there being so many different sizes and options to choose from, it is very important to understand which type of post cap is the most ideal solution for your desired application. With this being said, it is also extremely important for you to understand how to measure your posts, whether they are made of vinyl or just plain wood. Without this knowledge, you could easily purchase the wrong size or type of post cap. Don't worry though, we've got you covered through every step of the process!


Understanding & Measuring Your Post Cap

In order to find the post cap that will pair perfectly with your post, you must first understand the measuring process. The caps that we offer are labeled for use with the named post size, therefore technically each cap will measure slightly different than its actual dimensions. It should fit just snugly enough that it will prevent corrosion beneath, while also being able to grip and hold itself onto the top of the post.

When determining which cap is most ideal for your desired appication, it is important to first understand that there are two different types of caps offered: Internal and External. Internal caps have ridges that extend from the bottom side to fit perfectly inside the top of your post, while External caps extend past the edges of the post and grasp the outer edges of your post.

When looking at the differences between the two types of caps, the most important things to keep in mind are the type of posts you are using the caps with and the way you would measure the two different types. When measuring an external cap, you would measure it from the inner wall to the opposite side's inner wall. And when measuring an internal cap, you would measure from the outer edge of the ridge on the bottom side to the outer edge of the parallel ridge.

Measuring Your External Post Cap

As mentioned before, you must measure external post caps from one inner wall to the parallel side's inner wall. This is because external post caps are designed to sit directly on top of your post and fit just tightly enough that it prevents corrosion by stopping water from penetrating the surface of the post as well as allowing the cap to grip and hold itself onto the post without the use of any screws or adhesive.

Looking at the diagrams featured below, you can see the size differences in relation to the cap size and the actual measurements. Often times, an external post cap will look much bigger than the actual dimensions. This is because the caps will typically taper down into a smaller size on the bottom (see diagram on right), which is actually the post size that it is intended to be used with.

Measuring external post caps is a fairly simple process. It is essentially measuring from the inner edge of one of the post cap's walls to the opposite side's inner edge and it should consistently match with the corresponding post size.

*Scroll your mouse over the images above to see the inside dimensions and how to properly measure your External Post Cap before installation.

 

Measuring Your Internal Post Cap

Measuring internal post caps can be a little bit trickier when compared to the simplicity of external caps. When calculating internal post cap size, it is important to remember that this type of cap is going to have ridges on the bottom that will slide down and lock into your post, therefore the cap itself is going to look larger than the post size it is designed to be used with.

When determing which size of internal post cap to use, a common problem customers face is the fact that vinyl posts will have varying wall thinknesses. The wall thickness is how wide your vinyl post walls are and different types of posts will have different measurements of wall thickness. With this being said, it is crucial to determine your post wall thickness before determining which internal post cap will truly fit into your post.

To measure your internal post cap size, you will essentially do the same thing that was discussed in measuring an external post cap, however with these, you must measure from the outside of one of the bottom ridges directly across to the opposite side's outer ridge. If done properly, these measurments will accurately provide you with the information needed to pair the cap with its corresponding post size.

*Scroll your mouse over the images above to see the inside dimensions and how to properly measure your Internal Post Cap before installation.


Understanding & Measuring Your Post Size

Whether you have wooden or vinyl posts, it is crucial for you to first understand what size post cap you are in need of. If you are unsure about your post size, we will show you exactly how to measure your posts and determine which size post cap you need. However before measuring anything, there are several things that must be acknowledged.

Wooden Posts

When discussing and measuring wooden posts, it is essential for you to first understand the differences between Nominal and Actual sizes of the wooden posts. In the lumber industry, the identification system that is used specifies terms such as 4x4 and 6x6, however these measurements can be confusing if you don't know the difference between Actual and Nominal dimensions.

Nominal dimensions are the terms that the lumber industry uses. These would be the 4x4 and 6x6 measurements. Actual dimensions, on the other hand, are the true measurements of the posts. So technically a 4x4 will only measure 3-1/2" x 3-1/2" and a 6x6 would only measure 5-1/2" x 5-1/2".

This is because Nominally measured wood states the dimensions of the green, unfinished planks. However in the process of drying, wooden planks shrink significantly. On top of that, once the wood is dried and shrunk, it is then planed and shrunk even more in order to smooth the surfaces and make the post a single, consistent size. After the completion of this phase, the lumber is then considered "finished". This is where the confusion usually occcurs. A simple rule of thumb to keep in mind when finding your post size is that when the " (inch) symbol is used, these are the Actual measurements. Otherwise, the measurements would be considered Nominal.

This diagram shows you how a Nominally measured 4x4 wooden post actually measures 3-1/2" x 3-1/2".

This diagram shows you how a Nominally measured 6x6 wooden post actually measures 5-1/2" x 5-1/2".

Vinyl Posts

Vinyl posts are used with post caps more frequently when compared to plain wooden posts because not only do they look much better, they typically will last much longer and pair perfectly with your post cap. When using vinyl posts though, measuring can be somewhat tricky depending on what type of post cap you desire to use.

When measuring your vinyl post for an internal post cap, it is important to take note of the wall thickness. This is because depending on which type of vinyl post you are using and what the wall thicknesses are, you may need a different size internal post cap. To measure which size internal post cap you will need, measure your post from the inner wall of one side to the inner wall of the opposing side (as demonstrated below).

If you are measuring your vinyl post for an external post cap, measuring will be much more simplified. This is because external post caps sit on the outside of the post, therefor you need not worry about your post's wall thickness. Simply measure from the outer edge of your post to the outer edge of the opposing side (as demonstrated below). If done properly, these measurments will accurately provide you with the information needed to pair the post cap with its corresponding post size.

This diagram is for measuring Internal Post Caps and shows you that you need to accurately measure the length spanning from the inner edge on one side to the inner edge of the opposing side.

This diagram is for measuring External Post Caps and shows you that you need to accurately measure the length spanning from the outer edge on one side to the outer edge of the opposing side.


Index:

Post: Vertical column, usually 4" x 4" or 6" x 6", that supports either the deck or railing. Posts support the deck beams and transfer the weight of the deck to the concrete footings. Posts extending above the level of the deck can also serve as railing supports.
Post Cap: Vinyl cap placed on the tops of posts to provide a finished look and prevent water from penetrating the post. Available in a wide variety of styles and sizes.
Actual Dimensions: The exact measurements of a piece of lumber.
Nominal Dimensions: The identifying dimensions of a piece of lumber; nominal dimensions are larger than the actual dimensions.
Treated Lumber: Lumber that has been pressure-treated with an approved preservative under appropriate standards of the American Wood Preservers' Associations (AWPA) or American Society for Testing and Materials (ASTM).
Outer Diameter (OD): This is the measurement of the outside dimensions of your post, picket or post cap, including the wall thickness.
Inner Diameter (ID): This is the measurement of the inner dimensions of your post, picket or post cap, excluding the wall thickness.
Wall Thickness: This is the width of the walls in vinyl posts. These will vary depending upon which type of vinyl post you are using.