Is a 2 by 4 stud really 2 inches by 4 inches? How about a 2 by 6 or a 4 by 4? Well, once upon a time a 2 by 4 actually was 2 inches by 4 inches – maybe 100 years ago, long before modern lumber processing. Today all dimensional lumber sizes refer to “nominal” wood dimensions, which means the dimension of the wood before it was dried and planed to size.
So a 2 by 4 is the nominal size, while the actual size is 1½ inches by 3½ inches; a 2 by 6 is actually 1½ by 5½ inches; a 4 by 4 is 3½ by 3½. For dimensional lumber, 8 inches really means 7½, and 10 inches means 9½, and 12 inches means 11½.
In modern wood processing, lumber starts at the nominal larger dimension and then is kiln dried to reduce its moisture content. Drying the lumber causes it to shrink and possibly twist and bend. After drying, the lumber is planed to a final, uniform size.
One very uncommon exception occurs when green lumber is used for construction. In this case the pieces of lumber might actually be the larger dimension. The carpenter must take this into account and plan for shrinkage that will occur after the lumber is installed.
