Nominal versus actual size
Lumber is named by its rough, uncut dimensions, called nominal size, but the finished board you buy is smaller because it has been dried and planed smooth. A 2x4 is not two inches by four inches. In the US, it typically measures about 1.5 by 3.5 inches.
Common dimensional lumber, with typical actual sizes:
- 1x4: about 3/4 by 3-1/2 inches
- 1x6: about 3/4 by 5-1/2 inches
- 2x4: about 1-1/2 by 3-1/2 inches
- 2x6: about 1-1/2 by 5-1/2 inches
- 2x8: about 1-1/2 by 7-1/4 inches
Lengths are usually sold in 2-foot steps and are close to their stated length, though they may be a little over. Standards and local practice vary, so measure before you design, particularly if parts need to fit together.
Plywood has a similar quirk. A sheet sold as 3/4 inch may be a bit thinner, which matters when cutting dados or fitting parts into a groove.
Hardwood: rough thickness and quarters
Hardwood is often sold in a different system. Thickness is stated in quarters of an inch of rough lumber: 4/4 (pronounced four-quarter) is about 1 inch rough, 8/4 about 2 inches. After planing, 4/4 stock often ends up around 3/4 inch, depending on how much needs to be removed. Boards may be sold rough, with one or both faces planed, and widths and lengths vary rather than being standard. Always ask what surface the board has.
The board foot
Hardwood is commonly priced by the board foot, a unit of volume equal to 144 cubic inches. A piece 1 inch thick, 12 inches wide and 12 inches long is one board foot.
A worked example, using hypothetical dimensions: a rough board 1 inch thick, 6 inches wide and 8 feet long. Eight feet is 96 inches. Volume is 1 x 6 x 96 = 576 cubic inches. Divide by 144 and you get 4 board feet. Because nominal thickness is used for pricing, and boards lose material when planed and cut, buy more than the finished project needs. Many people add an allowance for waste, with a common range often quoted at roughly 15 to 25 percent, though it depends on the quality of the boards and the complexity of the design.
Softwood grades
Softwood grading generally falls into categories by purpose. Framing lumber is graded for strength and appearance, typically with names such as Select Structural, No. 1, No. 2 and No. 3, with lower numbers having more knots and defects. A grade stamp on the board usually shows the grading agency, the grade, the species group and the moisture condition. Stud-grade boards are sold for wall framing. Appearance grades are used where a clean surface is the goal, such as trim and finish material, and are sometimes labeled “select” or “clear.”
Higher grades usually cost more, so a cheaper grade is often a good option if you can cut around knots or the part will be hidden. Check each board anyway, because grade does not guarantee flatness or lack of warping.
Hardwood grades
Hardwood grading is typically based on the amount of clear, usable wood that can be cut from a board. In the US, the National Hardwood Lumber Association system is widely used. Grade names such as FAS (First and Seconds), Select and No. 1 Common describe how much of a board yields clear cuttings of a given size, with FAS the highest of those mentioned. Lower grades have more defects but can be a bargain for small parts, because you can cut clear pieces from between the flaws. Species, region and supplier differences mean that the same grade name can look different between sources.
Other labels worth knowing
- Moisture marks: a stamp such as “S-DRY” or “KD” indicates the dryness at the time of grading, but it can change in storage. Lumber acclimation is the way to deal with that.
- Treatment marks: treated lumber carries a tag with its intended exposure, which matters for outdoor use.
- Cuts of the log: flat-sawn, quarter-sawn and rift-sawn describe how the board came from the log, and affect stability and appearance.
- Surface: S4S means surfaced four sides, while rough-sawn is unplaned. Rough stock needs milling before use.
- Species names: a “whitewood” or “SPF” label can mean a mixed group of similar softwoods, not a single species.
Buying tips
Bring a tape measure and a short list of the actual dimensions you need. Sort through the stack when permitted, looking down each board for bow, twist and cracks, as in dealing with warped and twisted boards. Do not pay for a grade you will not use, and do not buy a lower grade you cannot work around. Understanding the label on the board is the cheapest way to avoid surprises when you start cutting.