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Why Isn't A 2X4 A 2X4?

Dimensional lumber is sized as newly-sawn lumber. As dimensional lumber is dried in the kilns it shrinks across the grain, so a 2X4 is no longer a 2X4 in size.

One of the most confusing things for the new woodworker to understand is how lumber is measured. Dimensional lumber, like the 2X4, 2X6, 2X8, Etc. is 1 ½ X 3 ½, 1 ½ X 5 ½, and 1 ½ X 7 ¼ inches. The new woodworker thinks that a 2X4 is 2”X4” but it is not. Why is this? Why do they call a 2X4 a 2X4 if it is only 1 ½ X 3 ½ inches? The truth is that the sizes given for dimensional lumber are “Nominal Sizes” because no two 2 X 4s (1 ½ X 3 ½) are exactly the same size. Why is that?

What is meant by “nominal”?

Dimensional lumber is sized by its rough-sawn size. It is the rough-size before the lumber is planed and dried. “Nominal” can also be read “Approximate”.

The size of dimensional lumber is based on newly-sawn lumber.

Dimensional lumber is cut from trees soon after they arrive at the saw mill. The trees, and thus the newly-sawn dimensional lumber is soaking wet. The sopping wet dimensional lumber is then sent to the ovens, the kilns, to be dried. As the moisture is removed from the wood, the cellulose structure shrinks. The cells shrink individually with the greatest shrinkage being across the grain. The length of dimensional lumber shrinks minimally, but the width and thickness of dimensional lumber shrinks significantly.

Softwood versus hardwood

The sizing of hardwood can be even more confusing than the sizing of softwood dimensional lumber. The size of hardwood depends on whether it has been surfaced on one side (S1S), two sides (S2) or surfaced four sides (S4S. Hardwood seldom comes in standard sizes but is measured and sold in board-feet. A board-foot is a cubic measurement computed by multiplying the length by width by thickness and dividing by 144. For example, a 1 inch by 4 inch by 72 inch board is equal to 2 board-feet.

The Waste Factor

When using the board-feet measurement you must calculate in a waste factor. Depending on the grade of hardwood being used the waste factor can vary from 15 percent to 30 percent. If the stock is high-grade, clear stock add about 15 percent to the calculated board-feet. If the stock is low-grade, rough-stock, add approximately 30 percent for waste.

In conclusion

If you need a 2X4 that is 2”X4” you are going to have to buy a 3X6 and cut it down to size.

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Comments (7)

Very informative. I had asked my dad the same question when he was building our house. I had to help him when I was 11 and 12 years old. I knew by looking it wasn't the size that it said it was.. and he said the same thing you said.

A friend of mine who owns a beautiful older home will say it was "built when a 2x4 really was a 2x4". Thanks for clearing up the confusion.

I don't use much lumber, but I have wondered why wood is always noticeably smaller than marked. Your explanation sure makes sense.

I once asked my father the same question years ago. Your explanation is very clear and concise, and will certainly clear up any confusion for the new woodworker.

Great article! I wondered about this for a long time.

I am saving this article for future reference. Thank you.

Another elaborate tip from the DIY guru on the net. Thumbs up, Jerry.

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