Understanding nominal wood sizes is essential for anyone involved in construction, woodworking, or home improvement. The terminology used to describe lumber dimensions can be confusing, especially when measurements from the planning mill differ from the stamped designations. This guide clarifies the standard naming conventions, explains why the gaps exist, and provides a practical reference for selecting the correct stock for your project.
The Origin of Nominal Sizing
Historically, wood was sold based on its rough-cut state directly from the log. A board labeled as "two by four" was exactly two inches thick and four inches wide. As sawmilling technology advanced, manufacturers began to plane and smooth the wood surfaces. This process removed material from all sides, reducing the final dimensions. To maintain a consistent marketing standard and simplify ordering, the industry adopted the original rough measurements as the "nominal" size, even though the actual physical dimensions were smaller.
Standard Dimensional Lumber Chart
The most common structural lumber follows a predictable pattern based on the nominal size. Below is a breakdown of the standard measurements for framing and finishing stock found in North America.
Why the Measurements Differ The discrepancy between the nominal and actual sizes exists for practical reasons. After the wood is milled, it is often left to dry, or "seasoned," in a kiln. This drying process causes the wood to shrink and warp slightly. Rather than adjusting the names constantly, the industry standardized the labels to the pre-drying dimensions. Furthermore, a "2x4" is initially cut slightly larger to allow for final planning and sanding at the construction site, ensuring the finished piece meets precise smoothness and uniformity standards. Board Foot Measurement
The discrepancy between the nominal and actual sizes exists for practical reasons. After the wood is milled, it is often left to dry, or "seasoned," in a kiln. This drying process causes the wood to shrink and warp slightly. Rather than adjusting the names constantly, the industry standardized the labels to the pre-drying dimensions. Furthermore, a "2x4" is initially cut slightly larger to allow for final planning and sanding at the construction site, ensuring the finished piece meets precise smoothness and uniformity standards.
