What are the actual dimensions of a 4×4 inch timber post ?

Solar deck post cap light is widely accepted and installed by homeowners, which can add guidance and illumination to the entire outdoor space. And will help your deck glow.

But before you make the decision to buy and install the solar post cap light ,there are some points you need to know:


First , you need to check the dimension of your posts, most of the homeowners installed the timber post, and the normal size you bought from the home improvement stores like HomeDepot or Lowes, is 4x4 inch or 6x6 inch post.  These two sizes are most used for garden and rail. But if you've ever carefully measured the actual dimension of these posts , you will find the problem.


For example as below, we find a review on amazon about the dimension issue of the post adaptor, and leave a 2 stars review. He finds the solar post cap lights he bought , did not match his posts.

According to his measurement , the 6x6 inch adaptor measures only 5-7/8 inch outside. And 3-1/2  inch on the inside for a 4x4 inch base.

That make the solar post cap light did not fit his vinyl caps very well. From the picture left , we can find the post cap light base is a little small than his vinyl post, and he is afraid that the rain may get into the post and damage the wood post inside. That is understandable for this worry.


So why the claimed 4x4 inch post cap and 6x6 inch post cap from the stores did not match the post well? Luckily the customer leave another photo let us find the clues.

From the photo you can find the vinyl post sleeve is standard 4x4 inch , but the lumber post inside is around 3-1/2 inch, we guess the lumber posts he bought from the stores, it was claimed of 4x4 inch.

See below specification of a  4 in x 4 in lumber post from HomeDepot:  we can find there are two dimensions, one is Nominal product width which is 4 in as claimed, and one is the Actual product width which is only 3.56 inch. We can find the actual dimensions is not same as the nominal dimensions for the lumber posts. That means the 4 in x 4 in lumber post you bought from the stores, the actual dimension is only 3.56x3.56 inch.

Now we can find the problem for the above review, the adaptor  for the solar post cap lights is based on the lumber post’s actual dimension, that is why the inside size of the base of the post cap light is around 3 1/2 inch to fit the lumber posts. And if the lumber posts are protected with a vinyl sleeve, the sleeve dimension of course is bigger than post, that make the post cap can not match with it.

So what are the nominal vs actual dimensions in lumber post ?


Nominal vs. Actual Measurements of Dimension Lumber


Nominal Size   Actual Size 
2 x 12 inches 1 1/2 x 11 1/4 inches (38 x 286 mm)
4 x 4 inches 3 1/2 x 3 1/2 inches (89 x 89 mm)
4 x 6 inches 3 1/2 x 5 1/2 inches (89 x 140 mm)
6 x 6 inches 5 1/2 x 5 1/2 inches (140 x 140 mm)


In North America, the lumber industry defines standard wood post sizes as 4x4 or 6x6. The outside dimensions of the standard 4x4 measure 3-1/2" x 3-1/2", and the outside dimensions of the standard 6x6 measure 5-1/2" x 5-1/2".

Likewise, how wide is a 2 by 4? A piece of surfaced (sanded smooth) 2x4 lumber actually measures 1½ inches thick and 3½ inches wide. In rough-cut condition, a 2x4 is slightly less than 2 inches thick and approximately 4 inches wide. When wood is milled from a rough to a smooth surface, it loses about ¼-inch from each of its four sides.

Also in North America, hardwood lumber is commonly sold in a "quarter" system, when referring to thickness; 4/4 (four quarter) refers to a 1-inch-thick (25 mm) board, 8/4 (eight quarter) is a 2-inch-thick (51 mm) board, etc.

What are the inside dimensions of post caps?

The inside dimensions of the standard 4x4 post cap measure 3-5/8", and the inside dimensions of the standard 6x6 measure 5-5/8". a little bigger than the actual 3-1/2 inch and 5-1/2 inch lumber posts.


So before you buy the 2021 best solar  post cap light, it is important to check the dimension of your posts , especially when these posts are protected by the vinyl sleeve post. In the market , most of the solar post cap lights can both fit the 4 x4 inch and 6x6 inch posts with an adaptor. But you need to know most of the solar post cap lights are just for timber posts, not for vinyl plastic posts, so you need to check or inquiry about that beore you buying them.


we offer with different kind of designs for these solar post cap lights, make you have more options. Especially the 2021 new solar flicker flame post cap light is popular. Contact us for more details.

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