In your case my guess is your fence posts were taller than the fence, so your wood fence posts should be in the ground at least 3 feet. When it is time to install the fence post, be sure it is in line with the other posts and it is plumb.It is also important that the fence post is centered in the hole.Start to add the crushed gravel in 5-inch lifts.our fence posts have a tremendous amount of force against them each time the wind blows. You may not think a 5-foot-tall fence is high, but in terms of wind load, it is substantial. To get an idea of what I am talking about, lift a 4x8 sheet of plywood up and take it outdoors on a windy day.