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.