Dimension lumber is lumber which has been cut and planed to a standardized width and height. The length can vary, with dimension lumber typically coming in an array of lengths for different tasks. This timber product is also known as dimensional lumber. The standardized measurements vary, depending on which nation the lumber is going to be sold in, but mills are required by law to use the same standards and measurements so that consumers know exactly what they are buying when they pick up lumber at the lumber yard. In the United States, for example, some common dimension lumber sizes include 2 inches by 4 inches (5.08 cm by 10.16 cm) and 2 by 6 (5.08 cm by 15.24 cm), also known as 2x4 and 2x6.
While the standard nomenclature used to refer to dimensional lumber might be perceived as a measurement of its actual size, this is not actually the case. The numbers refer to the wood when it is still "rough," before it has shrunk as a result of the drying and planing process. The actual standardized sizes are, as a result, slightly smaller than the numbers used to refer to the sizes.
Both hardwoods and softwoods are available as dimension lumber. Softwoods tend to be less expensive, as a general rule. It is also possible to purchase engineered wood products in standardized dimension lumber sizes. Depending on the type of wood, versions treated with chemicals to help the wood resist insects and rot are available along with dimension lumber which has not been treated.
Lumber mills tend to dry their wood to a standardized moisture level, which may be lower or higher than someone desires for a project. If the moisture level is too high, the wood needs to be allowed to dry further before it can be used. Using wet dimension lumber in a project means that the lumber will warp and cup as it dries, potentially compromising the project. For example, the framing of a house could bend, throwing the framing out of plumb.
In some regions of the world, very large sizes of dimension lumber are known as timbers, while smaller sizes are known as boards. This allows lumber yards and construction workers to divide the materials they work with into rough categories which help with organization and sorting. Cost of the product is usually based on length, with discounts available for people who buy large volumes of lumber and for those who do not require custom sizes.