Lumber dimensions are a bit misleading. When the board is first rough cut from the log, it is to the true measurement, such as a 4" x 6" post. However, the drying process and planning of the board reduces it to its finished dimensions. As such, our reference to a 4"x 6" post, is actually a 3-1/2" x 5-1/2" post. The wood post kit that comes with this hose reel, is designed to fit on posts whose exact width is either 3-1/2" or 5-1/2".