Because two-by-four is much easier to say.

Are you getting these questions from somewhere? They're pretty funny. And that's a good question.

Because 2" x 4" is the rough size. The finishing of the wood removes 1/8" from each long side, and 1/4" from each short side. At least that was the way it was when two-by-fours first came into being.

The standards for lumber that were set in the 1900s were, it was argued, put in place to keep unethical sawyers from cheating customers. At the same time, it established a wet measure, meaning that a 2"x4" was measured that way before the wood had dried. It also was suggested that the standards became minimums, so a 2"x4" wouldn't get any smaller than, say, 3 1/2" by 1 1/2". Also found this: This measuring method is probably the most recognized by the average person. We see this type of measuring method used in almost all "Do-It-Yourself" type stores that sell lumber, or any place selling lumber for construction purposes. We recognize such "sizes" as 2x4, 2x6, 4x4, 1x2, etc. This measurement refers to the thickness and width of the lumber and the length varies. In reality, these measurements are not a true measurement of the lumber thickness or width. The true measurement of a 2x4 is actually about 1.5x3.5. When the board is first rough sawn from the log, it is a true 2x4, but the drying process and planning of the board reduce it to the finished 1.5x3.5 size. The lumber is then sold as a "2x4" because the cost of the drying and machining are figured in...it is also much easier to refer to a board as a "2x4", rather than a "1.5x3.5".

It's because the rough cut board is 2" x 4". However, that which you buy is not a rough cut board. The boards are planed after they are sawn to given them smoother surfaces and they tend to shrink some as they dry out. Thus they are smaller than the stated measurements.