In the United States, softwood lumber is divided into three categories, based on the piece of wood's smallest dimension:
The stated length of a piece of lumber is its actual length, and is given in whole feet (usually, in increments of 2 feet). The thickness and width are nominal dimensions, as described below. The sizes are measured at the sawmill, but wood's dimensions change with its environment. They may be different in the lumberyard.
The most commonly available sizes are 1×2, 1×3, 1×4, 1×6, 1×8, 1×10 and 1×12.
Two-by-fours were originally two by four inches, unsurfaced. Surfaced, in the 1950s they were 1 ⁵/₈ by 3 ⁵/₈ inches, and have now shriveled to 1½ inches by 3½ inches. (Dimensions are slightly different for some species.)
No distinction is made between dry and green dimensions. Nominal dimensions of 5 and 6 inches may actually be at most ½ inch (13 mm) smaller. Nominal dimensions between 7 and 15 inches may actually be at most ¾ inch (19 mm) smaller. Nominal dimensions of 16 inches or more may actually be at most 1 inch (25 mm) smaller.
Dimensions are set by:
Voluntary Product Standard PS 20-05.
American Standard Softwood Lumber Standard.
Washington: U.S. Dept. of Commerce, September, 2005.
Available on the web at http://ts.nist.gov/Standards/Conformity/upload/ps20-05.pdf
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Last revised: 11 January 2007.