Lumber Dimensions Chart (Nominal vs. Actual Sizes)
Part of Decks, Fences & Roofing
Quick answer
A "2×4" is a nominal size, not the real one. The actual dimensions of a 2×4 are 1½ × 3½ inches; a 2×6 is 1½ × 5½ inches; a 1×4 is ¾ × 3½ inches; a 4×4 is 3½ × 3½ inches. The board is cut to the nominal size green, then dried and planed smooth, which shaves it down. Length is the exception — an 8-foot board is a full 8 feet.
Lumber is sold by its nominal size — the "2×4" or "1×6" stamped on the bin — but that's the rough-sawn size before the board is dried and surfaced. By the time it's planed smooth on all four sides (S4S), it's smaller. Knowing the actual sizes is the difference between a wall that lines up and one that doesn't.
Why a 2×4 isn't 2 by 4
A board starts at the full nominal size when it's freshly cut and wet. It's then kiln-dried (which shrinks it) and planed smooth (which removes more). The finished, surfaced board is what you buy. The amount removed is standardized by the American Softwood Lumber Standard (PS 20), so a 2×4 is the same 1½ × 3½ inches at any lumberyard in the country.
Dimensional lumber (2×) — actual sizes
| Nominal size | Actual size (inches) | Actual (mm) |
|---|---|---|
| 2×2 | 1½ × 1½ | 38 × 38 |
| 2×3 | 1½ × 2½ | 38 × 64 |
| 2×4 | 1½ × 3½ | 38 × 89 |
| 2×6 | 1½ × 5½ | 38 × 140 |
| 2×8 | 1½ × 7¼ | 38 × 184 |
| 2×10 | 1½ × 9¼ | 38 × 235 |
| 2×12 | 1½ × 11¼ | 38 × 286 |
Boards (1×) — actual sizes
| Nominal size | Actual size (inches) | Actual (mm) |
|---|---|---|
| 1×2 | ¾ × 1½ | 19 × 38 |
| 1×3 | ¾ × 2½ | 19 × 64 |
| 1×4 | ¾ × 3½ | 19 × 89 |
| 1×6 | ¾ × 5½ | 19 × 140 |
| 1×8 | ¾ × 7¼ | 19 × 184 |
| 1×10 | ¾ × 9¼ | 19 × 235 |
| 1×12 | ¾ × 11¼ | 19 × 286 |
Posts & timbers (4× and 6×) — actual sizes
| Nominal size | Actual size (inches) | Actual (mm) |
|---|---|---|
| 4×4 | 3½ × 3½ | 89 × 89 |
| 4×6 | 3½ × 5½ | 89 × 140 |
| 6×6 | 5½ × 5½ | 140 × 140 |
| 6×8 | 5½ × 7¼ | 140 × 184 |
The pattern is easy to remember: 2× lumber loses ½ inch on each face up to 6 inches nominal, then ¾ inch above that (so 2×8 and wider lose ¾). The nominal-to-actual jump is the same whether it's a 1×, 2×, or 4×. Lengths are honest — a board sold as 8, 10, 12 or 16 feet is that length or a hair over.
Where the actual size matters
- Stud walls: a 2×4 wall is 3½ inches thick, plus ½ inch of drywall each side = 4½ inches finished. A 2×6 wall is 5½ inches.
- Decking and spacing: a "6-inch" deck board covers 5½ inches, so you fit more rows than the nominal size suggests — see the deck calculator.
- Board feet: board-foot volume uses the nominal size by convention, even though the planed board is smaller. The board foot calculator handles it.
FAQs
What is the actual size of a 2x4?
A 2×4 actually measures 1½ inches by 3½ inches (38 × 89 mm). The "2×4" is the nominal (rough-sawn) size; the board loses material when it's dried and planed smooth.
What is the actual size of a 2x6?
A 2×6 is actually 1½ × 5½ inches (38 × 140 mm). Like all dimensional lumber, it's planed down from the rough-cut nominal size.
Why is lumber smaller than its name?
Boards are cut to the full nominal size while green, then kiln-dried and surfaced (planed) on all four sides for a smooth, straight, consistent board. Drying and planing remove material, and the finished sizes are standardized nationally by PS 20.
Are lumber lengths also smaller than nominal?
No. Only the cross-section (thickness and width) is reduced. An 8-foot board is a full 8 feet long — often a fraction over, never under — so length is the one nominal dimension you can take at face value.