Nominal Lumber Sizes VS Dimensional Lumber Sizes

It can be a confusing predicament the first time that you realize that the nominal lumber measurement, i.e.:  two-by-four, 4 inches by 4 inches, etc., does not match the true dimensions of the wood.  However, it is easy to understand nominal lumber sizes vs. dimensional lumber sizes once you understand the reasoning behind it, and then you can adjust your building projects accordingly.

Measurements of Lumber

Common sizes of lumber are referred to in terms of measurement. The most often-used board is referred to as a two-by-four, which refers to a board that is 2 inches high and 4 inches wide. The first number is always the height, or thickness, while the second number is the width. The length is irrelevant as a two-by-four board that is 8 feet long or 2 feet long is still referred to as a two-by-four.

Nominal Lumber Sizes

Common lumber sizes that are used in building homes, decks and gardening are two-by-four, 1 in by 6 inches, 4 inches by 4 inches and 2 inches by 6 inches. There is a wide array of lumber sizes available, but some have so many applications that they are the bread and butter of the lumber industry. Traditionally, the woods used for building and construction are soft woods, such as pine. When a pine log is sent to a wood mill to be cut into lumber, it is cut to the nominal size; a two-by-four really does measure 2 inches thick by 4 inches wide. However, the cut lumber that matches the nominal size is rough cut. It does not have the smoothness, or ‘dressed’ surface, of the lumber found in home improvement stores.

Dimensional Lumber Sizes

After the initial cut at the saw mill, rough cut wood is ‘dressed’, or planed flat and smooth on all sides. The finished board can lose from ¼ inch to ½ inch in thickness and from ½ inch to ¾ inch in width. So a two-by-four actually measures 1½ inches thick by 3½ inches wide. It is an industry standard to continue to use the nominal names when referring to pieces of finished lumber.