Dimensions … What is dimensional lumber? Lumber Dimensions. For example: a 2-by-4 actually measures 1 1/2 inches thick and 3 1/2 inches wide. 2x4s are not actually 2 inches by 4 inches. Lumber dimensions in widths and thickness are 1/2-inch less than the size. 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. If you are using rough-sawn, or "true dimension" timbers, however, 4 inches means 4 inches. However, the actual dimensions are 1 1/2 inches by 3 1/2 inches. It's important to remember this when purchasing lumber, because mixing nominal and actual dimensions … Dimensional lumber is cut and finished lumber (planed on all four sides) that comes in standardized sizes. Knowing lumber sizes in advance helps reduce waste. Here are the common sizes of lumber, and their actual sizes. 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