Lumber is generally sized according to thickness, as in the case of rough sawn lumber, or in specific widths, thicknesses and/or lengths as in the case of dimensional lumber. Here we will define each to help you better understand the sizing methods used when referring to lumber "size".
Dimensional Lumber: This measuring method is probably the most recognized by the average person. We see this type of measuring method used in almost all "Do-It-Yourself" type stores that sell lumber, or any place selling lumber for construction purposes. We recognize such "sizes" as 2x4, 2x6, 4x4, 1x2, etc. This measurement refers to the thickness and width of the lumber and the length varies. In reality, these measurements are not a true measurement of the lumber thickness or width. 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 planning 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".
The "Quarters" Sizing Method: In the case of rough sawn lumber, the "Quarters" sizing method is usually used by mills that sell rough lumber for woodworking purposes. The "Quarters" method refers only to the thickness of the wood as widths and lengths vary depending on the log the wood is cut from. Generally, a woodworker will plane the boards to the desired thickness and most likely rip the boards and glue them up into joined panels to get the desired width. Rough sawn lumber comes in "true" thicknesses as reflected by the "quarters" size. Lumber sized according to "Quarters" reflects the number of quarters of an inch thick the lumber is. To figure the thickness of a board referenced in "quarters" sizes, simply divide the second number (4) into the first number. The second number (4) means "quarters of an inch", or "quarters". So, a "4/4" board is four quarters, or 1 inch thick, an "8/4" board is eight quarters, or 2 inches thick, a "10/4" board is ten quarters, or 2.5 inches thick, etc.
Board Foot: Rough sawn lumber is usually sold by the "board foot" (bd. ft.). A board foot is equal to a piece of wood 12 inches long x 12 inches wide and 1 inch thick, or 144 cubic inches. To figure the board foot measurement of a piece of wood, multiply the length x width x thickness in inches, then divide by 144.