When you walk into a lumberyard and ask for a two by four, the last thing you probably think about is the actual dimensions of that board. Most people assume the names we use for dimensional lumber correspond to its final size. In reality, the industry standard names are simply labels for rough sawn wood that is later milled. The actual measurements of dimensional lumber are smaller than the names suggest, and this discrepancy is the source of frequent confusion for DIYers and builders alike.
The Origin of the Size Discrepancy
The system we use today dates back to the early days of commercial logging in North America. When trees were first cut, the wood was shipped as rough-sawn lumber, which is much thicker and wider than the finished product. Standardizing these random widths and lengths was inefficient for mills and builders. To solve this, the industry adopted nominal sizing, which essentially acted as a classification system rather than a promise of final dimensions. A "two by four" was categorized by its rough state, making it easier to communicate and price materials before it was planed and dried.
Actual Measurements You Need to Know The difference between the name and the reality becomes clear once the wood is dried and planed smooth. Kiln-drying and processing remove material from the surface, reducing the width and thickness. Below is a quick reference guide for the most common pieces of dimensional lumber used in residential construction. Nominal Size Actual Thickness Actual Width 2 x 2 1 1/2 inches 1 1/2 inches 2 x 4 1 1/2 inches 3 1/2 inches 2 x 6 finished width="true" 1 1/2 inches 5 1/2 inches 2 x 8 1 1/2 inches 7 1/2 inches 2 x 10 1 1/2 inches 9 1/2 inches 2 x 12 1 1/2 inches 11 1/2 inches 1 x 4 3/4 inches 3 1/2 inches 1 x 6 3/4 inches 5 1/2 inches 1 x 8 3/4 inches 7 1/2 inches Why the Size Shrinks
The difference between the name and the reality becomes clear once the wood is dried and planed smooth. Kiln-drying and processing remove material from the surface, reducing the width and thickness. Below is a quick reference guide for the most common pieces of dimensional lumber used in residential construction.
