Why Isn't a 2x4 Actually 2 Inches by 4 Inches?
The short answer
A 2x4 sold at a US home center measures about 1-1/2 inches by 3-1/2 inches. The "2x4" is a nominal dimension — the rough size of the board when it was first sawn from the log. Between the sawmill and the store shelf, the board is dried and then surfaced (planed smooth on all four sides), and both steps remove material. Drying shrinks the wood as it loses moisture, and planing removes the rough, fuzzy sawn surface. What's left is the actual dimension, and for softwood construction lumber those actual sizes are standardized: a 1x4 is 3/4 by 3-1/2 inches, a 2x6 is 1-1/2 by 5-1/2 inches, and so on. The physics of why wood shrinks as it dries — and why it shrinks more across the grain than along it — is covered in depth in the USDA Forest Products Laboratory's Wood Handbook, which is the standard reference for how lumber gets from tree to board.
Hardwood is sized differently
Hardwood dealers mostly don't use the 2x4 system at all. Hardwood thickness is quoted in quarters of an inch, rough-sawn: 4/4 ("four-quarter") means the board was sawn one inch thick, 8/4 means two inches. After surfacing, a 4/4 board typically finishes out around 13/16 or 3/4 inch. Widths and lengths are usually random — you buy whatever widths the boards came out at, rather than uniform sticks. This surprises people on their first lumberyard trip, and it's why hardwood is priced by volume rather than by the piece.
Board feet, in words
That unit of volume is the board foot: the amount of wood in a piece one foot long, one foot wide, and one inch thick. To figure a board's volume, multiply its thickness in inches by its width in inches by its length in inches, then divide by 144. Two useful habits follow. First, always calculate with the rough (nominal) thickness — you pay for the 4/4 inch, not the 3/4 that's left after planing. Second, buy 15 to 30 percent more volume than your cut list requires, because defects, warp, and grain-matching will eat the difference. Our guide to why boards warp explains where that waste factor comes from.
S4S, S2S, and rough
Boards are also labeled by how much surfacing they've had. Rough stock is straight off the kiln, bandsaw-textured, cheapest per board foot, and requires a jointer and planer to use. S2S is surfaced two sides (faces planed, edges rough). S4S is surfaced four sides and ready to cut — what home centers sell — at a premium for the milling. If you own no milling machines, that premium is usually worth paying.
One more label to check
While you're reading tags at the yard, you may see an FSC mark on some stock. That's a chain-of-custody certification about where the wood came from, not about its dimensions or grade — what the FSC label means covers it, and the certifying body itself, the Forest Stewardship Council, publishes the standards behind the mark.