Nominal vs. Actual Lumber Sizes

Why lumber is sold by a size it isn't

A "2x4" starts as a rough-cut piece that really is about 2 inches by 4 inches. It's then dried and planed smooth on all four sides (S4S), which removes about 1/2 inch from each dimension -- leaving the finished board at 1-1/2" x 3-1/2". The name "2x4" describes the size before milling, not the board you actually buy. This has been standard industry practice for decades, and every stud, joist, and piece of dimensional lumber at any lumberyard follows the same convention.

Why this matters for your project

Framing calculations (stud spacing, joist spans, wall thickness) use the actual dimensions, since that's the real wood in your wall or floor. Pricing and board-footage, on the other hand, traditionally use the nominal size -- so a "2x6" is priced and measured as a 2x6 even though you're holding a 1-1/2" x 5-1/2" board. Mixing these up in either direction throws off a material estimate.

Second tab: plywood & OSB actual thickness

Sheet goods have the same nominal-vs-actual gap. A nominal 1/2" plywood or OSB panel is commonly manufactured closer to 15/32" (about 0.469"), and nominal 3/4" is often closer to 23/32" (about 0.719"). This matters for anything relying on a tight tolerance, like flush-mounted subfloor seams or router-set door hinges -- measure your actual sheet before cutting mating parts.

Dimensions reflect the American Softwood Lumber Standard (PS 20) actual size for surfaced dry (S-DRY) dimension lumber, the convention used industry-wide in the US. Weight is an approximate figure for kiln-dried softwood framing lumber (~35 lb/ft³) and varies by species and moisture content.