Lumber Size Chart: Nominal vs. Actual

Lumber is sold by its nominal size -- the size it was cut at the sawmill, before drying and planing. A "2x4" starts life close to a true 2 in by 4 in, but the drying process shrinks it and the planing (smoothing) process shaves more off, leaving a smaller actual size once it reaches the lumber yard.

For most dimensional lumber, the rule of thumb is: subtract 1/2 in from each nominal dimension of 2 in or more, and 1/4 in from a nominal dimension of 1 in. A 2x4 (nominal) finishes at 1-1/2 in x 3-1/2 in (actual). A 4x4 finishes at 3-1/2 in x 3-1/2 in, not a true 4 in square.

This matters for framing math (stud spacing, header sizing), deck and fence post sizing, and anywhere you're calculating based on a board's real physical dimensions rather than its trade name.

Where these numbers come from

Standard nominal-to-actual dimensions for surfaced (S4S) dimensional softwood lumber, per common US milling practice. Some suppliers and rough-sawn lumber differ -- verify with your specific yard for anything precision-critical.