Nominal size in lumber refers to the idea that a piece of lumber is first cut to a size and then as it is prepared for use, it is still called by that original size. Planing will reduce the actual dimension as will reducing the moisture content (drying). This is why a 2×4 does not really measure a full two inches by a full 4 inches. Final sizes vary for different species and grades of wood. The following is a general chart to help in the selection of your material.