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 & 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 Vinyl Post Caps, sometimes known as Post Plugs have ridges extending from the bottom to fit perfectly inside the top of your post. 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.
- External Vinyl Post Caps extend past the edges of the post and grasp the outer edges of your post. When measuring an external cap, you would measure it from the inner wall to the opposite side's inner wall.
OD vs ID: Knowing the Dimensional Difference
While all of our vinyl post caps are "True to size" which means that a 4" Post Cap is going to fit on a 4" post, it's still important to know what it means when a Post Cap is displaying it's OD or ID size. OD, meaning Outer Dimension, is typically used when referring to the Outer Dimension of the Post that a Post Cap or any other type of fitting would be placed onto. For example if a Post Cap said that it fit was 2" x 3 1/2" OD, that would mean that it fits on a Post with Outer Dimensions of 2" x 3 1/2". Similarly, ID, meaning Inner Dimensions, is used for the inner hole of a Post and is used to determine the size of an Interior Fitting. For example, if an Interior Post Cap or Plug said that it fit a 2" Round ID Post, then you would need a 2" Round Interior Post Cap. Other manufacturers may vary as not all Inner Dimensions are the same, however at Vinyl Fence Fittings all of our Post Caps are as listed.
Inside Diameter is used for the Inner Dimensions of the Post, typically used for Interior Fitting Post Caps or Plugs.
Outside Diameter is used for the Outer Dimensions of the Post, typically used for Exterior Fitting Post Caps or Plugs.
Reminder: When measuring you're measuring the Post and NOT the Post Cap
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.
| Post Size (OD) | Cap Size (External) |
|---|---|
| 2" x 2" | 2" External Cap |
| 2 1/2" x 2 1/2" | 2 1/2" External Cap |
| 3" x 3" | 3" External Cap |
| 3 1/2" x 3 1/2" | 3 1/2" External Cap |
| 3 5/8" x 3 5/8" (4" Wood Post) | 3 5/8" External Cap |
| 4" x 4" | 4" External Cap |
| 4 1/4" x 4 1/4" | 4 1/4" External Cap |
| 4 1/2" x 4 1/2" | 4 1/2" External Cap |
| 4 1/2" x 4 1/2" (5" Wood Post) | 4 1/2" External Cap |
| 5" x 5" | 5" External Cap |
| 5 1/2" x 5 1/2" | 5 1/2" External Cap |
| 5 5/8" x 5 5/8" (6" Wood Post) | 5 5/8" External Cap |
| 6" x 6" | 6" External Cap |
| 6 1/2" x 6 1/2" | 6 1/2" External Cap |
| 8" x 8" | 8" External Cap |
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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.
| Post Size (Measured ID) | Cap Size (Internal OD) |
|---|---|
| 1 7/8" x 1 7/8" | 1 13/16" Internal Cap |
| 2 3/8" x 2 3/8" | 2 5/16" Internal Cap |
| 2 3/4" x 2 3/4" | 2 11/16" Internal Cap |
| 3 1/4" x 3 1/4" | 3 1/8" Internal Cap |
| 3 3/8" x 3 3/8" (4" Wood Post) | 3 1/4" Internal Cap |
| 3 3/4" x 3 3/4" | 3 5/8" Internal Cap |
| 4" x 4" | 3 7/8" Internal Cap |
| 4 1/4" x 4 1/4" | 4 1/8" Internal Cap |
| 4 3/8" x 4 3/8" (5" Wood Post) | 4 1/4" Internal Cap |
| 4 3/4" x 4 3/4" | 4 5/8" Internal Cap |
| 5" x 5" | 4 7/8" Internal Cap |
| 5 1/4" x 5 1/4" | 5 1/8" Internal Cap |
| 5 3/8" x 5 3/8" (6" Wood Post) | 5 1/4" Internal Cap |
| 5 3/4" x 5 3/4" | 5 5/8" Internal Cap |
| 7 3/4" x 7 3/4" | 7 5/8" Internal Cap |
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Understanding & Measuring Post Caps Based on Material
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.
Measuring for 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: The Standard 4x4 and 6x6 that the lumber industry uses.
Actual Dimensions: The True measurements of the posts. So for example, 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".
To better explain, 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.
Measuring for 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.
Measuring For Internal Post Caps
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.
Measuring For External Post Caps
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. If done properly, these measurements will accurately provide you with the information needed to pair the post cap with its corresponding post size.
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Frequently Asked Questions on Measuring Post Caps
How Do I Measure My Vinyl Fence Posts for the Correct Post Cap?
To measure your vinyl fence posts accurately, first determine whether you're using internal or external post caps. For internal caps, measure the inner dimensions of the post by calculating the distance from the inner wall on one side to the inner wall on the opposite side. This measurement accounts for the cap's ridges that fit inside the post. For external caps, measure the outer dimensions by spanning from the outer edge of one side to the outer edge of the opposite side. Accurate measurements ensure a snug fit, preventing issues like water infiltration or cap loosening.
What Is the Difference Between Internal and External Post Caps?
Internal post caps are designed to fit inside the top of the post, featuring ridges that slide into the post's interior. This design provides a seamless look and helps secure the cap without additional hardware. External post caps, on the other hand, sit over the top of the post, covering the exterior edges. They often have a tapered design to grip the post securely and are easier to install, especially when wall thickness varies.
Why Is Wall Thickness Important When Measuring for Internal Post Caps?
Wall thickness affects the internal dimensions of your vinyl post, which is crucial for internal post caps. Different vinyl posts can have varying wall thicknesses, altering the space available for the cap's ridges. Measuring only the outer dimensions without accounting for wall thickness can lead to selecting a cap that doesn't fit properly. Therefore, always measure the inner dimensions to ensure compatibility with internal post caps.
How Do I Measure for an External Post Cap?
Measuring for an external post cap involves determining the outer dimensions of your post. Use a tape measure to span from the outer edge of one side to the outer edge of the opposite side at the top of the post. This measurement should match the internal dimensions of the external cap, ensuring a snug fit. External caps are generally more forgiving with slight size variations, but precise measurements are still recommended for the best fit.
What Are Nominal vs. Actual Post Sizes, and Why Do They Matter?
Nominal sizes refer to the labeled dimensions of lumber, such as 4x4 or 6x6, but these are not the actual measurements. For example, a nominal 4x4 post typically measures 3.5" x 3.5". This discrepancy arises from the finishing process, where wood is planed down from its original size. Understanding the actual size is crucial when selecting post caps, as relying on nominal sizes can lead to mismatches and improper fits.
Can I Use the Same Measurement Techniques for Wooden Posts?
Yes, but with caution. Wooden posts also have nominal and actual sizes, and they can vary due to factors like moisture content and milling differences. Always measure the actual dimensions of your wooden posts, both for internal and external caps. This ensures that the post caps fit correctly, providing both aesthetic appeal and protection against environmental elements.
Vinyl Post Caps Glossary of Terms
| Name | Definition |
|---|---|
| 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. |
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Have questions about measuring your fence posts correctly? Our guide walks you through measuring post dimensions, understanding outside vs. inside dimensions, and matching your fence fittings with the right post sizes. Whether you're working with vinyl, wood, or metal posts, using accurate measurements is key to a secure and lasting installation. If you're unsure about your specific application or need help finding compatible hardware, we're here to help. Reach out to us today for expert advice and product recommendations tailored to your project.
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