To ensure optimal stability for your posts, it's advisable to excavate the post hole to a diameter that is three times the width of the post itself. For instance, a 4-inch wide post should be placed in a hole approximately 12 inches in diameter. The depth of the hole should correspond to one-third to one-half of the post's above-ground height. For example, a post supporting a 6-foot-tall fence should be set in a hole with a minimum depth of 2 feet. For setting the post, utilize gravel to establish the correct depth before proceeding to fill the remainder of the hole with concrete for a secure installation.