Dimensional Lumber

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) · Last reviewed Aug 28, 2026

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× 4 is initially cut to 2 by 4 inches. As the wood is kiln-dried to ≤ 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-inch nominal boards (1× 4, 1× 6): Actual thickness is 3/4 inch (0.75 in / 19 mm).
2-inch through 6-inch dimensions: Lose 1/2 inch (2 in → 1.5 in, 4 in → 3.5 in, 6 in → 5.5 in).
8-inch and larger dimensions: Lose 3/4 inch (8 in → 7.25 in, 10 in → 9.25 in, 12 in → 11.25 in).

Framing Layout and Stud Spacing

Standard residential wood stud framing uses 16 inches On-Center (O.C.) or 24 inches O.C. spacing. Because sheets of drywall and plywood measure exactly 48 by 96 inches (4× 8 ft), panel edges land precisely over the center of the 1.5-inch thick framing studs (48 div 16 = 3 stud bays), allowing shared nailing surfaces.