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By Chris | October 6, 2008 - 9:11 pm - Posted in Specs & Data

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.

lumber sizes chart - actual dimensions versus nominal dimensions

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.

five quarter lumber sizes - lumber dimensions

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.

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