Nominal vs. Actual Dimensions
Lumber is sold by nominal size (2×4) but the actual dimension after planing is smaller (1.5" × 3.5"). The nominal name is a rough-sawn historical convention.
In Detail
Common conversions: 2×4 = 1.5×3.5", 2×6 = 1.5×5.5", 2×8 = 1.5×7.25", 2×10 = 1.5×9.25", 2×12 = 1.5×11.25". 1× boards: 1×4 = 0.75×3.5". All span calculations use actual dimensions. The nominal system dates to when lumber was sold at rough-sawn size before planing.