Actual Size
The true measured dimensions of lumber after drying and surfacing, which are smaller than the nominal size. A nominal 2x4 has an actual size of 1-1/2 by 3-1/2 inches.
In-Depth Explanation
The difference between nominal and actual lumber sizes is one of the most confusing aspects of buying wood for beginners. Nominal dimensions reflect the rough-sawn size before drying and planing, while actual dimensions are what you measure after processing. A nominal 1-inch board measures 3/4 inch thick after surfacing, and a nominal 2-inch board measures 1-1/2 inches thick. Width reductions follow the same pattern: a nominal 4-inch width becomes 3-1/2 inches, a 6 becomes 5-1/2. These standardized actual dimensions are codified in the American Lumber Standard and apply to softwood dimensional lumber sold in the United States and Canada.
Usage Example
My plan called for a 1-1/2 inch thick shelf, so I bought a 2x8 knowing its actual size would be 1-1/2 by 7-1/4 inches.