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 difficult and time-consuming. With so many different sizes and options, it is essential to understand which type of post cap is the ideal solution for your desired application. With this being said, it is also imperative to understand how to measure your posts, whether they are made of vinyl or 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 and Measuring Your Post Cap

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


When determining which cap is ideal for your desired application, it is important first to understand that there are two different types of caps: Internal and External. Internal caps have ridges extending from the bottom 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 remember are the type of posts you are using the caps with and how 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

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 between the cap size and the actual measurements. An external post cap will frequently 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 the right), which is the post size that it is intended to be used with.


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

Wooden Post Cap External Installation Example

4 x 4 Square Vinyl Post Cap Measurement Diagram

6 x 6 Square Vinyl Post Cap Measurement Diagram

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


Measuring Your Internal Post Cap

Measuring internal post caps can be a little trickier 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 determining which internal post cap size to use, customers face a common problem: vinyl posts will have varying wall thicknesses. The wall thickness is how wide your vinyl post walls are, and different types of posts will have different wall thickness measurements. 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 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 measurements will accurately provide you with the information needed to pair the cap with its corresponding post size.


External Post Cap Installation Example

4 x 4 Internal Post Cap Installation Example

6 x 6 Internal Post Cap Installation Example

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


Understanding & Measuring Your Post Size

Whether you have wooden or vinyl posts, it is crucial for you first to understand what size post cap you need. 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, several things 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 planned and shrunk even more to smooth the surfaces and make the post a single, consistent size. After completing this phase, the lumber is considered "finished." This is where the confusion usually occurs. 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.

4 x 4 Wooden Post Nominal Measurements

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

6 x 6 Wooden Post Nominal Measurements

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 they look much better and typically will last much longer and pair perfectly with your post cap. When using vinyl posts, though, the 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 the type of vinyl post you are using and the wall thicknesses, you may need a different size internal post cap. To measure which size internal post cap you 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 outside the post; therefore, you need not worry about your post's wall thickness. Measure from the outer edge of your post to the outer edge of the opposing side (as demonstrated below). If done properly, these measurements will accurately provide you with the information needed to pair the post cap with its corresponding post size.

Vinyl Post Internal Measurement Diagram

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

Vinyl Post External Measurement Diagram

This diagram is for measuring External Post Caps and shows that you need to accurately measure the length spanning from the outer edge of 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 deck's weight to the concrete footings. Posts extending above the deck level 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.