Lumber Size Chart

In short Nominal lumber sizes are names, not measurements. A 2 × 4 measures 1.5 × 3.5 inches, a 1 × 6 measures 0.75 × 5.5, and a 2 × 12 measures 1.5 × 11.25. The reason is historical and mundane: boards are sawn oversize while green and rough, then dried and planed smooth on all four faces, and the name stuck to the rough green dimension rather than to what comes out the other end. The planing allowance is NOT constant, and that is the part worth knowing. A nominal 1 loses a quarter of an inch. Everything from 2 through 6 loses half an inch. Everything from 8 upwards loses three quarters. So the shortfall grows exactly twice across the range, and this page searches the table for where those breaks fall rather than reciting them. The consequence matters more than the trivia. A 2 × 4 has a nominal cross-section of 8 square inches and an actual one of 5.25 — it has lost 34% of its area. For anything carrying load, that is the number to design around, not the name on the label. Two warnings: these are US softwood dressed sizes and are a convention rather than a universal standard, and hardwood is a different world again, sold rough in quarter-inch thicknesses.