Why are physical wood dimensions different than the advertised dimensions? Why does a 2×4 measure 1-1/2” x 3-1/2” and not 2” x 4”? And what the heck is a nominal dimension? To answer these questions, we need to travel way back in time and look at the history of the 2×4 and how wood dimensions have evolved over time.
Wood Dimensions Prior to 1895:
Prior to 1895 standard lumber sizes varied greatly across the country. Since wood was generally supplied by local sawmills, there were no real standards across the country. However, the most common size was 2” x 4” for studs, while the typical dimensions for boards was 1 inch thick. With the advent of the railroad system, these sizes became more commonplace as larger mills began to ship wood products to neighboring states.
In these early years, the lumber was shipped in a “green” state, meaning the wood was still wet and therefore much heavier than dry wood. This led to the creation of kiln dried wood. By drying the wood and reducing its weight, the shipping costs were also lowered.
Does Kiln Drying Reduce Wood Dimensions?
While the process of kiln drying, will certainly reduce the wood dimensions, 2×4’s don’t shrink by a 1/2” so how did they get so small? As it turns out a few savvy sawmills started to reduce the size of 2×4’s ever-so-slightly to improve their profit margins. Additionally, mills started to plane lumber to further control the dimensions and provide a more consistent product. The planning process provided the added benefit of all-but-eliminating splinters, making the wood easier to handle.
In 1969 the U.S. Department of Commerce stepped in to establish unified wood dimensions from coast to coast. It’s a good thing they did, otherwise a standard 2×4 could be even smaller than it already is!
A Quick Visual Aid:
The term “nominal” refers to the original cut size before final processing. A nominal 2×4 actually measures 1-1/2” x 3-1/2”. How does the saying go? A picture is worth a thousand words! The same is true of a video. Watch this quick video on standard wood dimensions to gain an understanding of standard sizes of the dimensional lumber we use today.
The moral of the story is that you need to realize that the modern 2×4 actually measures 1-1/2” x 3-1/2”. This will be critical info to have in hand when you start framing your wall to accept your new windows.
Why does a 2 x4
not measure 2” x 4”?