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A 2x4 is 2x4

In the United States, softwoods like pine and redwood are sold by their "nominal" dimensions, while the actual dimensions for woods up to 7" is 1/2" smaller to account for planing and drying. So a pine board sold in the US as a 2x4 has a minimum size of 1 1/2" by 3 1/2".

This according to the woodworking FAQ at ftp://ftp.cs.rochester.edu/pub/archives/rec.woodworking/woodwork-faq . According to the FAQ, the corellation between the nominal and actual size of hardwoods sold in the US is even more complex. Weird.

In the tradition of boastful, macho construction workers, it's actually any plank with a 1.5" by 3.5" cross-section. When he tells you it adds up to six inches, you'll know it's more like five.

All joking aside, this is your standard wooden board, used for all sorts of things, available in pine, cedar, or other woods, salt treated, kiln dried, or plain, and a variety of lengths. Fortunately when you ask for an eight foot long two by four, you don't get seven and a half feet.


And now, the only (and therefore, worst) joke about two by fours, properly told in a redneck accent:

Q: What's a two b' four?

A: T'play inna polka band.

If you look at old construction, you'll see that the wood used to construct the frame was very rough. Modern two-by-fours are pre-planed, dropping their original dimensions by about one-half inch. Strangely enough, that happens to be the thickness of most gypsum boards (wallboards). By sheathing the walls, you bring it up to the proper thickness, and also the proper depth for batting insulation.

The name is a lie: as anyone who has ever designed and built something using 2x4s can tell you, a 2x4 is in fact 1.5x3.5 inches. Maybe you can doublethink yourself into using 2x4 2x4s, but you might find that your bolts are awfully short.

Nominal Size             Actual Size 
1 X 3                    .75 X 2.5
1 X 4                    .75 X 3.5
1 X 6                    .75 X 5.5
1 X 8                    .75 X 7.25
1 X 10                   .75 X 9.25
1 X 12                   .75 X 11.25
2 X 3                    1.5 X 2.5
2 X 4                    1.5 X 3.5
2 X 6                    1.5 X 5.5
2 X 8                    1.5 X 7.25
2 X 10                   1.5 X 9.25
2 X 12                   1.5 X 11.25 
2 X 14                   1.5 X 13.25
3 X 3                    2.5 X 2.5
3 X 4                    2.5 X 3.5
3 X 6                    2.5 X 5.5
3 X 8                    2.5 X 7.25
3 X 10                   2.5 X 9.25
3 X 12                   2.5 X 11.25
3 X 14                   2.5 X 13.25
3 X 16                   2.5 X 15.25
4 X 4                    3.5 X 3.5
4 X 6                    3.5 X 5.5
4 X 8                    3.5 X 7.25
4 X 10                   3.5 X 9.25
4 X 12                   3.5 X 11.25
4 X 14                   3.5 X 13.25
4 X 16                   3.5 X 15.25
6 X 6                    5.5 X 5.5
6 X 8                    5.5 X 7.5
6 X 10                   5.5 X 9.5
6 X 12                   5.5 X 11.5
6 X 14                   5.5 X 13.5
6 X 16                   5.5 X 15.5
6 X 18                   5.5 X 17.5
8 X 8                    7.5 X 7.5
8 X 10                   7.5 X 9.5
8 X 12                   7.5 X 11.5
8 X 14                   7.5 X 13.5
8 X 16                   7.5 X 15.5
8 X 18                   7.5 X 17.5
10 X 10                  9.5 X 8.5
10 X 12                  9.5 X 11.5
10 X 14                  9.5 X 13.5
10 X 16                  9.5 X 15.5
10 X 18                  9.5 X 17.5
12 X 12                  11.5 X 11.5
12 X 14                  11.5 X 13.5
12 X 16                  11.5 X 15.5
Source: http://www.engineersedge.com/commercial_lumber_sizes.htm
Confirmed using hardware store and yardstick, and to some extent, personal experience.
cbustapeck says re A 2x4 is 2x4: Hey, are you sure a 10 x 10 is actually 9.5 x 8.5? Shouldn't it be 9.5 x 9.5

Good point. Time to make a call.

Beep Boop Beep Bap Boop Beep Beep.
Ring. Ring. Ring.
Chipper hardware store employee: Hi! Well come to Ace Hardware, how may I direct your call?
Me: Hi, I have a question about the size of lumber.
Chipper: OK, hold on.
Silence........for about 3 minutes. Me: (hangs up) (redials)
Chipper hardware store employee: Hi! Well come to Ace Hardware, how may I direct your call?
Me: Hi, I have a question about the size of lumber.
Chipper: Uh...um...hold on.
Silence........for about 1 minute.
Ring. Ring.
Chipper: Wha....
Me: Hello?
Chipper: Oh. Let me try again.
Silence........for about 1 minute.
Ring. Ring.
Lumberjack Jack: YeeeeeeEEEllo! Lumber department.
Me: Hi, I have a question about the size of lumber.
Jack: YEAH! Sure, what is it?
Me: Exactly how big is a 10 x 10?
Brief silence.
Jack: You mean a 1 x 10 x 10 or a 2 x 10 x 10?
Me: No, no, a 10 x 10. As opposed to a 2 x 4. 10 x 10 x 16, or whatever.
Jack: What? I don't sell that shit. That's like a tree.
Me: Yeah, it's a big project.
Jack: I never heard of something like that.
Brief silence.
Jack: That's like a TREE, man.
Me: So you have no clue.
Jack: Wait, you mean like a 1 x 10, or what?
Me: Thanks. (hangs up)

So, cbustapeck, the answer to your question is nobody knows and it doesn't really matter anyway.

Tune in next week for: a call to the Department of the Interior.

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