Dimensional lumber is the term used for lumber that is cut to standard dimensions from a log or solid piece of wood. Dimensional lumber is often planed smooth and corners rounded so it is easy to use without getting splinters. The most commonly used dimensional lumber boards are the 2X4 and the 2X6. They come in various lengths that are usually cut in increments of 2 feet. Normal dimensional lumber lengths would be 8', 10', 12', 14', 16', 18' and 20'.
There are some lengths that are used a lot so you can buy dimensional lumber in lengths for these applications rather than having to buy the standard lengths in 2 foot increments and then cut them to the size you need. For example, you can get 2X4 and 2X6 dimensional lumber in the right length to be studs in walls when framing houses. If you want an 8 foot ceiling in a house you would use 2x4s and 2x6s that are 92 5/8 inches long. With standard construction practices you will end up with an 8 foot high ceiling, after sheetrock and flooring are installed, when you use 92 5/8 inch studs. If you are building a fence you can buy dimensional lumber that is 1X6 in 5 foot and 6 foot lengths which are standard fence building sizes.
The standard sizes for dimensional lumber are not exactly the size that they are called. For example, a 2X4 is not 2 inches by 4 inches. It is actually 1 1/2 inches by 3 1/2 inches. With the thickness of the blade cutting the lumber to size, and then the process of planing the lumber smooth, you lose some of the thickness. Plus the rough cut size is often not too precise so by taking a little off during the planing process, lumber mills can ensure that all the lumber is the same size. And I’m sure when lumber mills moved from the rough sawn lumber of olden days to the nice smooth stuff we have today there were some cost and profit issues that came into play. So a 1X4 is 3/4 inches thick and anything larger than that is going to be 1/2 inch smaller than the dimension stated. A 4X10 beam will actually be 3 1/2 inches by 9 1/2 inches.
Examples of dimensional lumber would be 2X4 studs in a wall, 4X4 posts, 4X10 headers or beams, 2X12 floor joists, or 1X6 fence boards.
Examples of things that are not considered dimensional lumber would be OSB, plywood, I-joists, glulam beams, or other engineered wood products.
