What are the Actual Two by Four Dimensions?

Two by fours – or more generally, the “two-bys” – are perhaps one of the most commercially common and used types of dimensional lumber in existence! But there is a nagging, puzzling mystery that can get some folks losing all kinds of sleep. And here it is: Why are two by fours NOT two inches by four inches? I mean, I thought this kind of deception was a thing of the past! Well ok, before we get too carried away, let’s dig a little bit and find out the facts surrounding this mysterious conundrum.

The Conundrum: Two by fours, two by sixes, two by eights, in fact, all two-bys and four-bys, are not actually their titles in inches, but that MINUS half an inch. This means that a two by four would actually be one and a half inches by three and a half inches. The same goes for all other two and four-bys. So let’s talk about why this confusion was so necessary and how it came to be.

When trees are first felled, they contain significant amounts of moisture – sometimes up to 50% of their weight in water! So you can probably imagine what happens when the freshly felled lumber begins drying out – they shrink! And yes, they can2 by 4 sample board shrink a lot. However, as much as they do shrink, the lumber is also planed after reaching a certain level of dryness to uniformize it for consumption.

Saw blades used for initial ripping of logs are manufactured to slice through the wood as quickly and efficiently as possible, and as such, have larger teeth, and leave a rather rough surface post-cut. This is another reason the lumber must be planed before it enters the market. And don’t forget that each dimension has two faces that will be planed, which means two reduced surfaces.

In North America and parts of Europe people go by inches, but in other parts of the world, the metric system is often used. For example, here in Japan, we talk in millimeters, avoiding centimeters due to possible confusion when dealing with decimals. A two by four converted to the metric scale is roughly 89 by 38 millimeters – 38.1 by 88.9 millimeters to be exact. Anyway, you can see why some people might question the importance of such an odd set of numbers.


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