Rough lumber comes from the sawmill without further cutting or shaping. It is usually sold in random lengths and widths and measured in board feet, a unit of 1 foot × 1 inch × 1 foot. It is available air-dried or kiln-dried. Air-dried lumber is carefully stacked and allowed to dry for several months, depending on thickness. It is used for some outdoor purposes, such as building sheds and fences. Kiln-dried wood is stacked and dried in moisture- and temperature-controlled kilns built for the purpose. It is then ready to be used for furniture-making or other woodworking uses.

Finished lumber is usually harvested from one of a few coniferous (needle-bearing) species such as pine, cedar, hemlock, fir or spruce. It’s kiln-dried, then planed and cut to predetermined sizes, primarily for use by the construction industry. When using customary measurements, the widths given are from before planing. The piece actually sold is smaller; a 2×4 for example, is actually only 1½ by 3½ inches after planing. Other stock is sized similarly. The lengths are actual sizes and are usually multiples of 2 feet. Sizes from 8 to 16 feet (8, 10, 12, 14, 16) are commonly available. Other sizes, smaller (4-, 6-, 7-feet) and larger (18-, 20-, 22-, 24-feet), are sometimes available.

Timber is cut in the forest in 24 foot lengths. At the mill it is again cut into three 8 foot lengths, an 8 foot and a 16 foot length, a 10 foot and a 14 foot length or two 12 foot lengths.

Lumber is also used to refer to plywood and other composite wood products.

Studs
The true measurement of a 2x4 is actually about 1.5x3.5. When the board is first rough sawn from the log, it is a true 2x4, but the drying process and planing of the board reduce it to the finished 1.5x3.5 size. The lumber is then sold as a “2x4” because the cost of the drying and machining are figured in ... it is also much easier to refer to a board as a “2x4,” rather than a “1.5x3.5.”

Examples of common sizes are 2×4, 2×6, and 4×4. The length of a board is usually specified separately from the width and depth. It is thus possible to find 2×4s that are four, eight, or twelve feet in length. Standard lengths of lumber are 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24 feet.

Dimensional Lumber
Dimensional lumber is a term used for lumber that is finished/planed and cut to standardized width and depth specified in inches. Sizes for dimensional lumber made from hardwoods varies from the sizes for softwoods. Boards are usually supplied in random widths and lengths of a specified thickness, and sold by the board-foot (144 cubic inches).

Also, hardwood lumber is commonly sold in a “quarter” system when referring to thickness. 4/4 (four quarters) refers to a one-inch thick board, 8/4 (eight quarters) is a two-inch thick board, etc. This system is not usually used for softwood lumber, although softwood decking is sometimes sold as 5/4 (actually one inch thick).