Contrary to its name, a 2x4 does not actually measure 2 inches by 4 inches. When the board is first rough sawn from the log, it is a true 2x4, but the drying process and planing of the board reduce it to the finished 1.5x3.5 size. This is due to the fact that the lumber is pre-cut to standard sizes, and the true measurement of a 2x4 is actually about 1.5 inches by 3.5 inches.