Lumber Dimensions Table
Why is a two-by-four called a “two-by-four” if it’s not two inches by four inches? Well, when the lumber is first cut from the log, the piece after that rough cut has the approximate width and depth (the nominal dimensions) by which the piece will be referred to from then on. At that point, a two-by-four is, in fact, about two inches by four inches.
The next step in the production process is to either air-dry or kiln-dry the green lumber, in order to reduce the moisture content of the wood. This causes the lumber to shrink, reducing its actual dimensions, and ensuring that the lumber doesn’t shrink significantly after it’s finished, sold, and used.
After that, the pieces are planed, smoothed, and have their corners rounded, reducing the actual dimensions even further, as well as allowing the producer to be certain that the final product has the dimensions required by industry standards (in the case of a two-by-four, that’s 1.5” x 3.5”). To learn more, see American Softwood Lumber Standard.