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Assembling a Butt Joint Many beginning woodworkers are confused about the way dimensional lumber (softwood) is sized. When you walk into a home center or lumber yard and buy a 2x4x8, logic would seem to dictate that it should measure 2" by 4" by 96" (8').

Aah, yes. But that would make sense!

Lumber manufacturers typically cut a tree into dimensional lumber very shortly after the tree is felled. Then, the newly-sawn (but soaking wet) lumber is kiln-dried until it reaches the desired moisture level. As lumber dries, it shrinks (as the moisture in the wood is reduced, the wood cells shrink, particularly across the grain). While the 8' length won't change much as the wood dries, the 2" width and 4" height (cross-section of the grain) will shrink considerably.

Because of this shrinkage, a typical 2x4 will usually measure out to around 1-1/2" x 3-1/2". These numbers can vary slightly, but modern lumber manufacturers are pretty adept at delivering consistently-sized supplies of dimensional lumber.

Hardwood and plywood sizing use completely different sizing conventions. In Understanding Wood Sizing, learn how hardwoods and plywood are sized, as well as a more thorough explanation of softwood sizing. Once you understand precisely how these materials are sized, you can design your woodworking projects appropriately.

(c) 2010 Chris Baylor licensed to About.com, Inc.

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Comments

November 17, 2010 at 12:22 pm
(1) Shawn Vincent says:

This is only half of the story.

In addition to shrinkage when drying, the “rough” lumber is also planed down to smooth it. This planing process results in smaller lumber yet, and allows lumber manufacturers to conform to standards regarding nominal versus actual lumber dimensions.

For a good discussion on this, see Wikipedia: http://en.wikipedia.org/wiki/Lumber#Dimensional_lumber

November 13, 2011 at 6:27 pm
(2) Rusty44 says:

Why are signs posted 2 x 4 instead of 1 3/4 x 3 1/2 then? I could care less what the wet dimensions were to begin with, when I am looking for a particular size, say 10″ wide, that’s what I want not a piece that is 9 1/4. The sign says 10″ that’s what I want to buy. Post it as 9 1/4, I can live with that. I just want to know why the signs misrepresent the product. No other industry gets away with that.

June 20, 2012 at 4:10 am
(3) Steve says:

If the Lumber manufacturers are pretty good with how much the wood will shrink, then why are they not Cutting the green lumber slightly larger, so that the shrunken lumber would conform very closely to the given dimensions? Its total BS in my opinion, either write down exact dimensions, or make sure that lumber is very close to the dimension. Oh btw, green lumber when I bought in the store, was already cut to the smaller dimensions. The industry is just BSing us all

April 9, 2014 at 12:14 pm
(4) matthew Ambrose says:

Money. At the end of the day it’s how much money “we” can save. We being the lumber mills.

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