Woodworking Tip: Dimensional Lumber Size Vs. Actual Lumber Size

As you get started with woodworking, it is important to note that the "nominal" sizes for the lumber are actually different than their actual sizes. You often hear a stud referred to as a 2x4, which means it was 2 inches by 4 inches at one time. The reason they refer to lumber by their nominal size is because that is the size the wood was when it was cut at the lumber mill. That does not take into account the shrinkage that happens to the wood when it is dried and the material that is removed when it is planed and sanded smooth. In general, to get lumbers actual size, subtract 1/4 inch from pieces one inch, subtract 1/2 inch from pieces between one and six inches, and subtract 3/4 inch from pieces that are more than six inches. See the tables below I found on Wikipedia. 

When looking at engineered wood such as MDF, particle board, plywood, OSB, LVL, etc., the dimensions given are the actual dimensions.

The hardwood at Home Depot or Lowes, which is S4S (planed on all four sides), is the same as the softwood chart above. When getting hardwoods at an actual lumber yard, they are different than the softwood chart above. The actual dimensions are slightly smaller than the nominal dimensions too. Lumber from a lumber yard is typically finished on only one or two sides (the other sides are rough and will have to be cut, planed and sanded). The thickness depends on if the hardwood is surfaced (or planed) on one side or on two sides. The dimensions below are if you buy the lumber from an actual lumber yard (not a Home Depot or Lowes).