The Fence Post! Different depths for different Post types!

Augered Post Hole!

        Fences are made of many different materials, some are of solid panels that catch the wind, and some allow the wind to flow through, so it must allow that the depths of their posts are all set to different depths also. I have lived and operated a Fence Company in 2 different places. Northern Tennessee and Southern Indiana. The "Mean" Frost or Freeze line was 3 inches in the Volunteer State and 6 inches up here in Evansville, Indiana. The only Building code that applies to installing Fence Posts state that 1/3 of the above ground length of the post for a 6' fence must be in the ground. That's it! That's all it says! Lucky for you, w have been digging holes and setting posts forever. The statement above about 1/3 of the post just delineates that for a 6 foot tall Wood fence you need to have 2 Feet in the ground, - not in concrete necessarily, just set in the ground. For 8' Wood Fences we do 32"inches. 
Setting Posts, in general: 
        Holes may be dug be any method, or tool. The best tools to get the results that are required is the Auger or a set of post hole diggers.
Both devices dig nice circular tube shaped holes. When you have dug down to the proper depth for the hole usage, use the post hole diggers the wallow out the edges around the bottom of the hole to an approximate bell shape. This makes the hole wider at the bottom. We try to make them about 2 inches wider but there is no hard and fast rule - just wider. Fence post's, when not set in Bell shaped holes are subject to be forced upward during the annual freeze / thaw cycle. That's how you see some Fences all wobbly and leaning back and forth after a period of time. This is called "Post Heaving", or "Frost Heave". When the ground freezes, or I should say when the moisture in the ground freezes, it expands. This expansion is a very strong force, and over a few winters the post may literally be heaved several inches upward if not set correctly. The next step is to tamp the bottom of the hole down flat, to a good hard, solid foundation. We like to use a tamping bar for this. A tamping bar is a 4' long steel pry bar weighing about 30 lbs with a rounded, flat surface on one end used to tamp. The hole must be tamped to ensure the post will not settle, that way the height of the fence will remain steady through out it's life. Next pour about 2" to 4" of gravel, or pebbles
 of the hole. Having the gravel will assist in water drainage in the post hole. This in turn prevents the bottom of the post from rotting. After the hole is set mix your concrete according to directions. Level the post and pour the concrete. Stop the concrete approx 4" for the top of the surface. This also will assist in the prevention of post heaving as the hardest frozen portion of the ground the part nearest the surface will not grip the concrete and force it upward. In fact it will work just opposite. The harder frozen surface - the 4 inches of soil will be tight against the post and actually holding the concrete down, where as if the concrete went all the way to surface the physics would act against the concrete and try to force it upward. We like to leave the post holes open for a day or 2 to allow them to dry prior to raking the dirt up against the posts.  For the different Fences and there heights please refer to this chart on the depths and hole widths to set your posts.

Fence Height:                Hole Depth:                       Hole widths:

ChainLink:
4' Chain link                   16" inches                           8"
5' Chain link                   18"                                       8"
6' Chain link                   24"                                      8"
 
Ornamental Aluminum:

4' and 5'                          18"                                      8"
6'                                     24'                                       8"

Wood and Vinyl Fences: 
3 and 4' Picket                20"                                     8"
5' and 6'                          24"                                     10"
8'                                      32"                                     12"