When visiting your local home center or woodworking supplier, you'll notice that typical lumber stock comes in varying standard wood sizes. Additionally, softwood sizing differs from hardwood sizes, the latter of which is most commonly sold by the board foot. While the numbers used to describe the typical dimensional lumber may seem logical, they can be a bit deceiving, as you'll soon discover.
Why one-inch isn't really one-inch:
Everybody has heard of a 2x4 (pronounced "two by four"), but few people realize that the actual height and width of a 2x4 is really somewhere close to 1 1/2" x 3 1/2" depending on dryness of the material and milling methods.
Similarly, a 1x (pronounced "one-by") is only about 3/4" in thickness. Wood shrinks when it is dried, so lumber mills adjust their tools accordingly. Because wood shrinks laterally (across the grain of the wood rather than with the grain), the length of a piece of stock is only minimally affected by this shrinking, so an 8' 2x4 when milled at the sawmill is usually very close or even slightly longer than 96-inches when it is available for purchase at your local home center or lumberyard.
Most softwoods, used in home construction and for general purpose woodworking, come in 1x and 2x sizes. In the top half of the chart shown, the Name columns display the "advertised sizes" and can be compared to the typical actual sizes in the corresponding column to the right. For instance, a 2x6 typically measures out at 1-1/2" x 5-1/2" in actual size. a 2x12 board would be far closer to 1-1/2" thick x 11-1/4" in width.
Hardwood sizing can be a bit more confusing, not only because of the varying measurement metrics used to price the boards, but also because it often depends on whether the stock is surfaced on one side (S1S) or on two sides (S2S). Notice in the Hardwood Sizing section of the chart (for dimensional hardwood lumber) that a one-inch piece of stock will typically measure 7/8" if it is surfaced on one side, but 13/16" if surfaced on two (opposite) sides.
Hardwoods rarely come in standard dimensions like softwoods. Instead of finding a 2x6 in hardwood vareties, you'll find that suppliers sell hardwoods in random varieties measured by the board-foot.
Additionally, hardwood may be sold in quarters. Each quarter refers to a 1/4-inch in thickness, meaning, a 5/4 board is roughly 1-1/4". If your project calls for a piece that is exactly one-inch thick, you'll want to purchase a 5/4 board and mill it down to the proper size using a surface planer.
Plywood us available in 4'x8' sheets, but once again, thickness sizes can be deceiving. The most common sizes of plywood are 1/2-inch and 3/4-inch, but in actuality, this corresponds to 15/32-inch and 23/32-inch in thickness respectively.
Plywood is graded in A, B, C and D grades, depending on the sanded finish of the two sides of the sheet, A being the smoothest. A sheet is graded on both sides: for instance, a sheet of BC plywood is grade B on one side and C on the other.