The fundamental difference between nominal size and size (often referred to as actual size) in the context of lumber lies in the manufacturing process.
The simple answer, as derived from industry standards, is that nominal size refers to the dimensions of lumber before it has been dried and planed smooth (surfaced) on all four sides. The actual size (or just 'size' in this comparison) refers to the final dimensions of the board after these processes are complete.
Understanding Nominal vs. Actual Size
Lumber begins as rough-sawn timber cut from a log. The initial cut dimensions are close to the nominal size. However, two key steps reduce the size:
- Drying: Wood shrinks as it dries. Lumber is typically dried in kilns to a specific moisture content, which reduces its overall volume.
- Planing (Surfacing): To create smooth, uniform pieces suitable for construction, the rough-sawn board is planed on all sides. This process removes material, further reducing the dimensions.
Therefore, the nominal size is essentially a historical or reference size, while the actual size is the precise measurement of the finished product you purchase and use.
Based on the SPIB Blog reference:
- Nominal measurements were derived from a board's size before it has been dried and planed smooth (surfaced) on all 4 sides.
- Actual measurements are the final dried and surfaced size.
Why the Difference Exists
This historical system originated when lumber was often sold rough-sawn and green (undried), making the nominal size closer to the actual dimensions at the time of sale. As manufacturing processes evolved to include drying and surfacing for improved stability and usability, the finished dimensions became consistently smaller than the original rough-sawn size. The nominal size system persisted as a convenient way to refer to standard lumber dimensions, even though the actual size differs.
Practical Implications
Understanding this difference is crucial for construction and woodworking projects:
- Accurate Planning: You must use actual dimensions when calculating material needs, cutting pieces, and ensuring proper fit.
- Compatibility: Fasteners, connectors, and other building materials are often designed to fit the actual dimensions of standard lumber.
Common Examples
Here's a look at some common nominal sizes and their corresponding actual sizes for dimension lumber (like pine, spruce, fir):
| Nominal Size | Actual Size |
|---|---|
| 1x4 | 3/4" x 3 1/2" |
| 1x6 | 3/4" x 5 1/2" |
| 2x4 | 1 1/2" x 3 1/2" |
| 2x6 | 1 1/2" x 5 1/2" |
| 4x4 | 3 1/2" x 3 1/2" |
| 4x6 | 3 1/2" x 5 1/2" |
| 6x6 | 5 1/2" x 5 1/2" |
Note: These are standard North American dimensions for softwood lumber. Actual sizes can vary slightly based on specific standards and manufacturers.
In summary, the nominal size is a larger, historical descriptor based on pre-manufacturing dimensions, while the actual size is the true, smaller dimension of the finished, dried, and planed lumber.
