Dimensional Lumber Sizes (Nominal vs Actual)
Direct Answer
Dimensional lumber sizes under the American Softwood Lumber Standard (DOC PS 20) define the difference between rough-sawn 'nominal' green lumber designations and finished 'actual' Surfaced 4-Sides (S4S) dry dimensions. A standard 2×4 measures exactly 1.5 × 3.5 inches (38 × 89 mm) following planing and kiln drying.
Source: DOC PS 20-20 (American Softwood Lumber Standard / ALSC)
Specs
| Governing Standard | DOC PS 20-20 / NIST / American Lumber Standard Committee (ALSC) |
| Surfacing Condition | S4S (Surfaced 4 Sides) Kiln-Dried (KD19, moisture content ≤19%) |
| 2×4 Actual Dimensions | 1.5 × 3.5 inches (38 × 89 mm) |
| 2×6 Actual Dimensions | 1.5 × 5.5 inches (38 × 140 mm) |
| 2×8 Actual Dimensions | 1.5 × 7.25 inches (38 × 184 mm) |
| 2×10 Actual Dimensions | 1.5 × 9.25 inches (38 × 235 mm) |
| 2×12 Actual Dimensions | 1.5 × 11.25 inches (38 × 286 mm) |
| 4×4 Actual Dimensions | 3.5 × 3.5 inches (89 × 89 mm) |
| 6×6 Actual Dimensions | 5.5 × 5.5 inches (140 × 140 mm) |
The 1/2-Inch and 3/4-Inch Sizing Rule
The discrepancy between nominal and actual lumber dimensions originates in historical sawmilling. When a green log is rough-sawn, a $2\times 4$ is initially cut to $2\text{ by } 4\text{ inches}$. As the wood is kiln-dried to $\le 19\%$ moisture content and planed on all four sides (S4S) to create smooth, splinter-free framing boards, it loses dimension according to standard rules:
- $1\text{-inch nominal}$ boards ($1\times 4, 1\times 6$): Actual thickness is $3/4\text{ inch}$ ($0.75\text{ in} / 19\text{ mm}$).
- $2\text{-inch through } 6\text{-inch}$ dimensions: Lose $1/2\text{ inch}$ ($2\text{ in} \rightarrow 1.5\text{ in}$, $4\text{ in} \rightarrow 3.5\text{ in}$, $6\text{ in} \rightarrow 5.5\text{ in}$).
- $8\text{-inch and larger}$ dimensions: Lose $3/4\text{ inch}$ ($8\text{ in} \rightarrow 7.25\text{ in}$, $10\text{ in} \rightarrow 9.25\text{ in}$, $12\text{ in} \rightarrow 11.25\text{ in}$).
Framing Layout and Stud Spacing
Standard residential wood stud framing uses $16\text{ inches On-Center (O.C.)}$ or $24\text{ inches O.C.}$ spacing. Because sheets of drywall and plywood measure exactly $48\text{ by } 96\text{ inches}$ ($4\times 8\text{ ft}$), panel edges land precisely over the center of the $1.5\text{-inch}$ thick framing studs ($48 \div 16 = 3\text{ stud bays}$), allowing shared nailing surfaces.