Actual lumber sizes are a pain to keep up with. I find myself having to refer to charts from time time and I deal with lumber all the time. I get asked about actual versus nominal lumber sizes on a regular basis. Most people know that a 2×4 is actually 1.5″x3.5″ but the measurements are different for the large boards like 2×8′s and 2×12′s.

Actually, the more common term is “lumber dimensions“, which I have written about before, but people often ask about “lumber sizes” so that is how I am explaining it here. Your choice of search terms is probably why you found this article.

Lumber Sizes

Here is a chart to clear up the confusion about 1x, 2x, and 4x nominal lumber sizes versus actual lumber sizes including the equivalent metric lumber sizes. This lumber sizes chart applies to treated and untreated pine construction grade lumber.

This chart applies to the lumber sizes of “quarter” measurements. The nominal lumber sizes are said as “five-quarter by four” or “six-quarter by six” etc. These are not all that common but you can usually find “five-quarter” decking whose actual dimension is 1″x5.5″. You might hear the 1/4 sizes used more commonly among more mature - no, more experienced - builders and lumbermen.

Timber Sizes

Lumber cut 5 inches or thicker is generally classified as timbers. Timbers are usually “rough cut” to actual sizes. In other words, what you see is what you get. A 6×6 is 6″x6″, a 10×10 is 10″x10″ and so forth. When you buy timbers in smooth, or S4S (Smooth 4 Sides) the sizes are usually 1/2″ smaller. In this case, a 6×6 would be 5.5″x5.5″.

Post Sizes

Round stock sizes can get a little complicated but we will keep it as simple as possible here. A thorough discussion including large poles requires getting into the differences between poles and pilings and classes of utility poles and what you are using them for and it goes on and on so…so for the purpose of this article, I will stick to small post sizes.

Small posts are usually measured by the top size (the little end). So, if you want a 4″ top x 8′ long fence post, you would ask for a “four inch – eight” post. The line between post sizes and poles is a fuzzy one but after about ten or twelve feet long, whatever it is that you want usually becomes a pole. If you are using it in water to support a structure it is probably a piling, which is used upside down and measured by the butt (the big end)… and see how it easy it is to get complicated when discussing poles?

If you want square posts make sure you are clear about that when you ask for “posts” as “posts” are usually considered to be round.

Lumber Sizes Questions?

If you have any questions about lumber sizes, let me know with a comment.

The life span of your treated posts posts matters. Before spending thousands, or tens of thousands, of dollars on a fence it is nice to have a better idea of how long you can expect your investment to last.

So, to answer the question: How long will my treated posts last?

According to the Southern Pine Council you can expect properly treated posts to last many decades. They site a study by USDA Forest Service’s Forest Products Laboratory saying:

Test stakes of treated wood have been buried in the ground at various locations, stretching from the Mississippi Delta to the Canadian border. Data analysis indicates that CCA-treated Southern Pine stakes in place since 1938 have shown no failures at chemical retention levels of 0.29 pounds of preservative per cubic foot of wood, or higher.

Most treated posts are treated to a retention of .40 but you should always ask – just to be safe.

Here’s a great pdf from the USDA with expected life spans for various species of treated posts including a comparison of the life spans of treated and untreated posts (see page two).

If you want a guarantee that your posts will last you can get treated posts coated at the ground line from American Pole and Timber. I mentioned these posts before in How to Build a Fence that Lasts because I have seen them up close and they are tough. They claim that posts coated at the ground line with their poly coating will last fifty years.? In reality, the posts should last 150 years because the ground line is the source of infestations and the place where decay begins.? If that is protected, you don’t have much else to worry about.

The bottom line is that the life span of properly treated posts should be at least 20 years and can be easily extended to 50+ when installed and used in normal conditions (not in water or along the coast, for instance) .? If you choose the right materials, your grandchildren won’t even have to deal with building another fence.