Lumber dimensions: nominal vs actual sizes
By Stephen Morrison, Editor Last reviewed
A 2×4 is 1½ × 3½ inches. Here's the full size chart from the US lumber standard, a size lookup, and a board-foot calculator.
Quick answer: lumber is sold by its nominal size, but the board you take home is smaller. It's been dried and planed. A dry 2×4 is 1½ × 3½ in, a 2×6 is 1½ × 5½ in, a 1×6 is ¾ × 5½ in and a 4×4 is 3½ × 3½ in.
Common lumber sizes
| Nominal | Actual (dry) | Typical uses |
|---|---|---|
| 2×4 | 1½ × 3½ in | Wall studs, rails, blocking |
| 2×6 | 1½ × 5½ in | Joists, rafters, headers, stair stringers |
| 2×8 | 1½ × 7¼ in | Joists, rafters, headers, stair stringers |
| 2×10 | 1½ × 9¼ in | Joists, rafters, headers, stair stringers |
| 2×12 | 1½ × 11¼ in | Joists, rafters, headers, stair stringers |
| 1×4 | ¾ × 3½ in | Trim, shelving, fence pickets |
| 1×6 | ¾ × 5½ in | Trim, shelving, fence pickets |
| 4×4 | 3½ × 3½ in | Posts |
| 5/4×6 | 1 × 5½ in | Deck boards |
The full chart: PS 20-25 Table 3
The standard lists thicknesses and widths separately. Any thickness can be paired with any width.
| Nominal | Dry | Green |
|---|---|---|
| 1 | ¾ | 25/32 |
| 5/4 (1¼) | 1 | 1-1/32 |
| 2 | 1½ | 1-9/16 |
| 3 | 2½ | 2-9/16 |
| 4 | 3½ | 3-9/16 |
| Nominal | Dry | Green |
|---|---|---|
| 2 | 1½ | 1-9/16 |
| 3 | 2½ | 2-9/16 |
| 4 | 3½ | 3-9/16 |
| 6 | 5½ | 5-5/8 |
| 8 | 7¼ | 7½ |
| 10 | 9¼ | 9½ |
| 12 | 11¼ | 11½ |
Timbers (nominal 5 in and thicker) follow their own rule: 5 and 6 in are ½ in under nominal, 7–15 in are ¾ in under when dry, and 16 in and up are 1 in under when dry.
Why actual sizes matter
- Deck boards: rows are counted with the actual 5½ in width, plus the gap. Our deck calculator does this.
- Fence posts: a 4×4 fills a 3½ × 3½ in square of the hole. The fence calculator subtracts it from the concrete.
- Walls: a 2×4 wall is 3½ in of framing. With ½ in drywall on both sides it's about 4½ in thick.
Common questions
Why is a 2×4 not 2 by 4 inches?
The name is the nominal size, roughly the size of the rough-sawn piece. Lumber is then dried and planed smooth (“dressed”), which takes it down to the standard minimum dressed size: 1½ × 3½ in for a dry 2×4.
What is the actual size of a 2×6?
1½ × 5½ in (dry). A 2×8 is 1½ × 7¼ in, a 2×10 is 1½ × 9¼ in and a 2×12 is 1½ × 11¼ in.
What is the actual size of a 4×4?
3½ × 3½ in (dry). That’s the post size our fence calculator subtracts from each post hole when it works out concrete.
How thick is 5/4 decking?
The standard’s minimum dry dressed thickness for nominal 1¼ (5/4) lumber is 1 in, and the Southern Forest Products Association lists 5/4 radius-edge decking at 1 in thick and 5½ in wide for a 5/4×6.
What are “dry” and “green” sizes?
Under the standard, dry lumber has been seasoned or dried to a maximum 19% moisture content; green lumber is wetter than that. Green pieces are dressed slightly larger because they shrink as they dry.
How do I calculate board feet?
Multiply the nominal thickness (in) by the nominal width (in) by the length (ft), then divide by 12. A 2×4 that’s 8 ft long is 2 × 4 × 8 ÷ 12 = 5⅓ board feet. Board feet always use nominal sizes, not actual.
Sources
- NIST, Voluntary Product Standard PS 20-25, American Softwood Lumber Standard (Table 3; §2.2 board measure; §2.7 and §2.12 dry/green)
- Southern Forest Products Association, Standard Sizing (incl. 5/4 radius-edge decking)
Formulas and figures last checked against these sources on. Spotted an error? Tell us and we'll fix it.