A 2×4 is one of the most commonly used pieces of wood in the world. It is used for framing houses to building sawhorses and everything inbetween. You would think, given the name, a 2×4 will measure 2 inches by 4 inches. That is not the case. Let’s look at the actual dimensions of a 2×4 and why this is the case.

Dimensions of a 2 x 4

The actual dimensions of a 2 x 4 are 1.5 inches by 3.5 inches. The 2×4 measurements are the dimensions it’s cut to at the mill. After the wood is dried and treated it shrinks to 1.5 inches by 3.5 inches. These are the dimensions you see at the store.

Nominal vs Actual size

When we talk about lumber, it is referred to as nominal size and actual size. There are some key differences between these two distinctions that you should know before going to your hardware store.

Nominal size is the size the wood is cut to at the mill. When a piece of wood is cut at the mill it is still wet. This is easier on the blade cutting it and just makes for a smoother process. Prior to the wood being sold in the store, it has to dry out. In this process, it shrinks down. So in regards to the 2×4, at the mill it is cut to 2 inches thick and 4 inches wide.

Actual size refers to the size that the wood shrinks down to and is sold at. After being cut at the mill, wood goes through a long drying period. It can take up to a year for a single 2 x 4 to dry out after being cut (this will vary and depends on the type of wood used).

A 2×4 is actually 1.5 inches thick and 3.5 inches wide. That is what you will see at your hardware store.

How moisture affects the size of wood

Moisture plays a huge role in the size of wood. Wood is a very porous material which allows it to absorb lots of water depending on the surrounding humidity and temperature.

When a piece of lumber absorbs water, it swells and becomes thicker and wider. Conversely, when it loses water, it shrinks and becomes thinner and narrower. This means that if you are using a 2 by 4 in a project that requires precise dimensions, you may need to account for changes in moisture content over time. This is most commonly seen on decks that sit out in the sun all day. You will often see boards shrink a couple months after being built. It is not uncommon to have to replace a couple boards because of this.

The picture above is of my deck. As you can see, the board with the X on it clearly has shrunk much more then the others. This can be seen with the increased gaps (shown by the arrows).

There are many tools out there to help you account for the size variations of wood that may occur for your given project.

Length of a 2 x 4

In addition to its thickness and width, a 2 by 4 also has a length dimension. The length of a 2 by 4 can vary depending on the needs of the project. For example, a standard 2 by 4 that is sold at a hardware store is usually 8 feet long.

The length of wood is much more variable then the thickness and width. The reason for this is because the height and width are more important in determining the wood’s strength and stiffness. A wood’s strength is primarily determined by its cross-sectional area. This is because the fibers in wood are oriented along the length of the tree trunk, which means that the properties of the wood are more dependent on the cross-sectional area (thickness and width) than on the length.

Also, it is much easier for you to change the length of a piece of wood then it is to change the thickness or width.

Above is a 2×4 that was cut down to 2.5 feet from an 8 foot length originally. For this I used my Dewalt circular saw.

Why is lumbar sold as its nominal length?

You may be asking yourself if a 2×4 measures at 1.5 inches by 3.5 inches why is it not sold as a 1.5×3.5? This is a fair question.

To answer this, we have to take a brief look at the history of wood making.

Dating back prior to WWII the demand for wood was not anywhere near where it is today. The process of making wood could be a lot slower at that time. Because of this, wood was air dried (which can take up to 1-2 years). Wood that’s air dried in general does not shrink as much because it can contain up to 20% of its moisture. Therefore, a 2×4 at this time was at or very close to 2 inches by 4 inches, hence it was given the name 2×4 (makes sense!).

Due to significant population increases post WWII the demand for lumber has increased drastically. With that, there is also a higher demand on quality. Consumers wanted good quality wood fast. Lumber mills did not have time for this long air drying process. That is when the first kiln was developed. A kiln allowed for much quicker and controlled hardening and drying of the wood.

In order to improve the quality of the wood at that time saw mills started using planes to smooth out the wood. This allowed for very precise smooth pieces of wood to be sold. So between the kiln drying and use of planes for smoothing, the wood lost some size, but the names never changed.

Actual sizes of commonly used pieces of wood

As you can imagine, the 2×4 is not the only piece of wood that has an actual size and a nominal size. All wood sold at your hardware store has a similar change in size. The good part is that the changes are pretty uniform throughout.

There is a general rule of thumb when it comes to wood shrinkage:

• Wood less than 2 inches: will loose 1/4 inch in width and or thickness
• Wood between 2-6 inches: will loose 1/2 inch in width and or thickness
• Wood greater than 6 inches: will loose 3/4 inch in width and or thickness

So for example: A piece of wood with nominal dimensions of 1″ by 4″ will shrink down to 3/4 inch by 3.5 inch (it’s actual size).

Below is a helpful chart you can use for some of the most commonly used pieces of wood:

Conclusion

• Nominal vs actual size
• How moisture affects the size of wood
• Length of a 2 x 4
• Why is lumber sold at its nominal length
• Actual sizes of commonly used pieces of wood

I hope this was helpful. If so, please let me know in the comments!