Posts holes should be dug deeper than the frost depth (sometimes called the frost line) for your geographic area. Check your local weather information to find out how deep the frost penetrates the ground in your area.
Also, posts need to be installed to a depth that prevents them from being pushed over. A long post can act as a powerful lever arm, and be popped out of the ground if it is not installed deep enough. The following guidelines should help you decide on the proper depth. Be aware that soft, sandy or wet soil is easy to disturb. You may need to conduct a test to determine the proper depth of the post-hole.
The following guidelines should help you to decide on the proper post-hole depth. These depths may have to be increased if the soil is soft, sandy, or wet; or if the local frost line is deeper than these guidelines.
POST LENGTH (FEET) POST HOLE DEPTH (INCHES)
If you cannot dig your post-holes as deep as the guidelines suggest, you may have to pour concrete in the post-hole after the post has been placed in the hole. This will help to stabilize the post.
Normally we do not recommend pouring concrete in the post-hole if you can follow the guide lines above. It is an added cost that you do not need to incur.
After the post has been placed in the post-hole, use a level at several places around the post to insure that it is plumb. Then begin to place the dirt back into the hole. Tamp the dirt every six inches as you back fill the post-hole.