Dimensional lumber actual size – Why the confusion?

Did you know that 2×4 is not actually 2 inches by 4 inches. I guess you did, since you’re reading this post. But did you know why? It’s actually not that complicated, and no, it’s not conspiracy from the lumber companies to give you less wood that you expected. But before we get into the “how & why”, let me tell you the dimensional lumber actual size:

Nominal size (in inches)Actual size (in inches)Actual size (in mm)
1×23/4 x 1 1/219 x 38
1×33/4 x 2 1/219 x 64
1×43/4 x 3 1/219 x 89
1×63/4 x 5 1/219 x 140
2×21 1/2 x 1 1/238 x 38
2×41 1/2 x 3 1/238 x 89
2×61 1/2 x 5 1/238 x 140
2×81 1/2 x 7 1/438 x 184
2×101 1/2 x 9 1/438 x 235
4×43 1/2 x 3 1/289 x 89
4×63 1/2 x 5 1/289 x 140

Dimensional lumber actual size chart

Why is a 2×4 not 2 inches by 4 inches?

Rough cut boards
Rough cut boards

This one will require a bit of a history lesson. Back in the olden days, photos were black and white, bars didn’t have Wi-Fi and raw lumber was cut to the nominal size. So a 2×4 was cut to be 2 inches thick and 4 inches wide. Quite simple right?

However, by the time the wood would dry and it was planed and ready to be sold, it was actually more close to today’s actual size. Keep in mind that things were less standardized back then, so don’t be surprised if you find a 2×4 stud in an old house that is actually 2 inches by 4 inches. Or something similar but not quite…

Nowadays, even the rough cut boards are smaller than their nominal size, because the improvements in the lumber industry allows for greater precision. Therefore, that 2×4 you bought at Lowe’s was likely never truly a 2×4.

What about the length, is that a lie too?

Nope, length is OK.

Wood shrinkage affects the dimensions across the grain (thickness and width in this case) a lot more than it affects the dimensions along the grain (length). Other than that, if you think about it, there is much more processing involved in getting the boards nice, straight and flat lengthwise than it takes to get them to the right length.

One less measurement to worry about…

What does this all mean to you?

A lot of beginners make the mistake of assuming that a 2×4 is actually a 2 inches by 4 inches. And it’s not a weird thing to assume, right? I surly know I thought so when I first heard the term.

Therefore, if you want to build a 2-inch thick tabletop out of 2x4s, you are going have to take the 3 1/2 side and get it down to 2 inches. Or just accept that your tabletop is going to be 1 1/2 inches and move on with your life.

But what about framing?

Wall framing
Wall framing

Luckily, framing is pretty standardized these days. If you follow the standard 16 on center approach, you’re gonna be fine. Most supplies, from siding to insulation are made to follow that standard so it usually all falls into place.

That doesn’t mean you should get too comfortable though. You still have to make sure everything falls into place. Your project – your responsibility.

But in the end, don’t worry about it too much

While this can be somewhat of a shock to new woodworkers, you’ll soon get used to it. Before you know it, you won’t even think about it and 1 1/2 by 3 1/2 will become your new unit of measurement.

When did you first find out that actual size doesn’t match nominal size? Is there something I failed to mention in this article? Be sure to let me know in the comments below.

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