Understanding nominal size wood is essential for anyone involved in construction, woodworking, or home improvement. This term refers to the standardized dimensions given to lumber pieces before they undergo the milling and drying process. When you purchase a board labeled as two by four, its nominal size is 2 inches by 4 inches. However, the actual dimensions are smaller due to the subsequent planing and finishing procedures.
What is Nominal Size Wood?
Nominal size serves as a universal label for lumber that simplifies ordering and communication. Historically, these measurements represented the rough-sawn dimensions of the wood. After cutting, the material is surfaced on all four sides (S4S), which reduces its thickness and width. Consequently, the actual size becomes approximately 1.5 inches by 3.5 inches for a two by four. This discrepancy between the label and the final measurement is a standard practice across the North American lumber industry.
The Difference Between Nominal and Actual Size
The distinction between nominal and actual dimensions is crucial for accurate project planning. If you rely solely on the nominal size, you risk assembling a structure that does not fit correctly. For instance, framing a wall with studs spaced on 16-inch centers requires accounting for the actual width of the studs, which is 1.5 inches. Ignoring this detail will throw off your measurements and lead to material waste or structural inconsistencies.
Common Examples of Dimensional Lumber
1x4: Nominal 1 inch by 4 inches; Actual 3/4 inch by 3 1/2 inches.
1x6: Nominal 1 inch by 6 inches; Actual 3/4 inch by 5 1/2 inches.
2x4: Nominal 2 inches by 4 inches; Actual 1 1/2 inches by 3 1/2 inches.
2x6: Nominal 2 inches by 6 inches; Actual 1 1/2 inches by 5 1/2 inches.
4x4: Nominal 4 inches by 4 inches; Actual 3 1/2 inches by 3 1/2 inches.
Why Does This Shrinkage Occur?
The reduction in size happens for practical reasons related to wood anatomy. Raw lumber contains a significant amount of moisture and has an uneven surface. The milling process removes the rough outer layers (the bark and sapwood) to reveal the stable, inner wood grain. Removing this material naturally reduces the width and thickness. The industry standard allows for this predictable shrinkage, ensuring that builders can work with consistent final dimensions.
Navigating Moisture Content and Measurement
Wood is a hygroscopic material, meaning it absorbs and releases moisture based on the humidity of its environment. The nominal size is often referenced when the wood is at a standard moisture content of 19%. However, if the wood is installed in a very dry environment, it may shrink further. Conversely, in high-humidity settings, it might swell. For critical applications, using a tape measure to verify the actual thickness of the wood in your specific environment is the best practice to ensure precision.
International Standards and Confusion Not all countries use the same system of nominal sizing. While North America uses inches, many European countries sell dimensional lumber in metric nominal sizes, such as 60mm x 40mm. These sizes are close to the North American 2x2 or 2x4 but are not always exact matches. If you are sourcing wood internationally or mixing materials from different regions, always convert and verify the actual measurements to avoid costly mistakes in your build. Best Practices for Ordering and Cutting
Not all countries use the same system of nominal sizing. While North America uses inches, many European countries sell dimensional lumber in metric nominal sizes, such as 60mm x 40mm. These sizes are close to the North American 2x2 or 2x4 but are not always exact matches. If you are sourcing wood internationally or mixing materials from different regions, always convert and verify the actual measurements to avoid costly mistakes in your build.
