Understanding Wood Sizes in Softwood Lumber
When you visit a local home center or woodworking supplier you'll find that typical lumber stock comes in bearings would standard sizes. Softwood sizing differs from hardwood sizes and the latter is most commonly sold by the board foot. You can use numbers to describe the typical dimensional lumber but it also can be deceiving.
Why 1 inch isn't really 1 inch
Everybody's heard of a 2 x 4. Few people realize that the actual height and width of the 2 x 4 is really closer to 1.5" x 3.5" depending on the dryness of the material in the milling method.
Similarly a one by is only about three-quarter inch in thickness. It shrinks when it is dry so lumber mills adjust their tools accordingly. Wood shrinks laterally across the grain rather than with the grain. The length of a piece of stock is only minimally affected by the shrinking so when an 8 inch 2 x 4 is milled at the sawmill is very close to 96 inches when it is available for purchase at your lumberyard.
Most softwoods that are used in home construction, come is 1X and 2X sizes. Typically the advertised sizes can be compared to the actual sizes and the corresponding column. For instance a 2 x 6 typically measures at 1.5" x 5 1/2 inch in actual size. A 2 x 12 board is closer to 1 1/2 inch thick by 11 1/4 inch width.
Hardwood sizing is more confusing because of the varying measurement metrics used to price the boards. It also depends on whether the stock is surfaced on one side S1S on two sides S2S.
The hardwood sizing section on a chart will typically measure 1-inch equals 7/8 if it is surfaced on one side, but 13/16" if surfaced on two sides.
Hardwoods rarely come in standard dimensions like softwoods. Instead of finding a 2 x 6 and hardwood varieties, you'll find that suppliers sell hardwoods in random varieties measured by the board foot.
Additionally hardwood is sold in quarters. Each quarter refers to 1/4 inch in thickness, meaning a 5/4 board is roughly 1 1/4 inch. If your project calls for a piece that is exactly 1 inch you want to purchase a 5/4 board and mill it down to the proper size using a surface planer.
Plywood is available in 4' x 8' sheets, but again the thickness can be deceiving. The most common sizes of plywood or 1/2 inch and three-quarter inch but in actuality this is more likely to be 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 is the smoothest and rated on both sides. If it says BC then the plywood is grade B on one side and C on the other.