The lumber industry defines standard wood post sizes as 4x4 or 6x6. The outside dimensions of the standard 4x4 measure 3-1/2 x 3-1/2, and the outside dimensions of the standard 6x6 measure 5-1/2 x 5-1/2.
Subsequently, question is, how wide is a 2 by 4? A piece of surfaced (sanded smooth) 2x4 lumber actually measures 1½ inches thick and 3½ inches wide. In rough-cut condition, a 2x4 is slightly less than 2 inches thick and approximately 4 inches wide. When wood is milled from a rough to a smooth surface, it loses about ¼-inch from each of its four sides.
Similarly one may ask, what size is 4/4 lumber?
Also in North America, hardwood lumber is commonly sold in a quarter system, when referring to thickness; 4/4 (four quarter) refers to a 1-inch-thick (25 mm) board, 8/4 (eight quarter) is a 2-inch-thick (51 mm) board, etc.
Why is a 2x4 not 2 inches by 4 inches?
Dimensional lumber is the general name for framing lumber. Now, most timber is milled and planed to give it a little more of a finished look, and a little more of a consistent size and profile. Because of this extra milling, a 2x4 no longer measures a full 2 inches by four inches.