Lumber Dimensions
Authority: ALSC (American Lumber Standard Committee) / PS 20
Dimensional softwood lumber nominal sizes differ from actual surfaced (S4S) dimensions due to drying and planing: 2x4 is actually 1.5 x 3.5 inches (38 x 89 mm), 2x6 is 1.5 x 5.5 inches (38 x 140 mm), and 4x4 is 3.5 x 3.5 inches (89 x 89 mm).
| Nominal Size (in) | Actual Surfaced Size (in) | Actual Surfaced Size (mm) | Board Feet per Linear Foot |
|---|---|---|---|
| 1 x 4 | 0.75 x 3.50 in | 19 x 89 mm | 0.333 BF/ft |
| 2 x 4 | 1.50 x 3.50 in | 38 x 89 mm | 0.667 BF/ft |
| 2 x 6 | 1.50 x 5.50 in | 38 x 140 mm | 1.000 BF/ft |
| 2 x 8 | 1.50 x 7.25 in | 38 x 184 mm | 1.333 BF/ft |
| 2 x 10 | 1.50 x 9.25 in | 38 x 235 mm | 1.667 BF/ft |
| 2 x 12 | 1.50 x 11.25 in | 38 x 286 mm | 2.000 BF/ft |
| 4 x 4 | 3.50 x 3.50 in | 89 x 89 mm | 1.333 BF/ft |
| 6 x 6 | 5.50 x 5.50 in | 140 x 140 mm | 3.000 BF/ft |
Why Nominal Sizes Don't Match Actual Sizes
Before industrial milling took over, dimensional lumber genuinely measured what its label said -- a rough-sawn 2x4 was close to 2 inches by 4 inches. That changed as sawmills mechanized through the late 19th and early 20th centuries. Mills began surfacing boards smooth on all four sides (S4S) for a more consistent, splinter-free finish, which shaved material off every face. Drying removed still more: green, unseasoned wood loses meaningful thickness as moisture leaves the cell walls, shrinking the board further after the initial cut.
By the early 1900s, mills across the country were surfacing lumber to inconsistent actual sizes even while selling it under the same nominal label, which made it difficult for builders to specify materials with any confidence. Through the 1920s, the lumber industry worked with the U.S. government to formalize uniform surfaced dimensions, a process that evolved into the American Softwood Lumber Standard (PS 20), still maintained today by the American Lumber Standard Committee (ALSC). The nominal number survived as a trade label even after the actual board shrank well below it.
Board Feet and the Standard
PS 20 does more than fix surfaced sizes -- it also standardizes lumber by moisture content, since a board keeps shrinking slightly as it dries even after milling. The standard sets separate size categories for kiln-dried and green (unseasoned) lumber so the trade can distinguish boards that will still shrink further from boards already at their final size.
The board-foot measurement in the table above is a volume unit, not a size: one board foot equals a piece of lumber 12 inches by 12 inches by 1 inch (144 cubic inches), used industry-wide to price and order lumber regardless of the actual cross-section being cut.