The depth of your post holes--and how well the posts are anchored--are the most important factors in the stability of your fence. If you do not get the post holes deep enough there is a greater potential that your fence can be blown over. Before digging your fence post holes, call 8-1-1, the national Call-Before-You-Dig hotline, to start the process of having all underground utility and communication lines marked on your property. This will help you to avoid interrupting services or potentially putting yourself in danger.
Essential Tools and Materials
Post lengths for fences start at 8 or 10 feet for a 6-foot-tall fence. They are commonly four-by-fours, which measure approximately 3 1/2 by 3 1/2 inches, or six-by-sixes, at around 5 1/2 by 5 1/2 inches. These are the approximate dimensions for standard milled lumber; rough-sawn posts tend to be larger and measure closer to their nominal sizes. The typical panel height on a privacy fence is 6 feet, and the standard panel length is 8 feet.
Post holes can be dug with a post-hole digger and shovel, or you can use a power auger. An auger is a drill-like machine designed to dig out post holes in hard ground.
The width of your post holes should be a minimum of 10 inches wide, if you're using four-by-four posts and filling the holes with concrete. Dig your holes about 8 inches in diameter if you're using the same posts and backfilling with soil and gravel. One rule of thumb is to make the holes three times wider than the post width, or about 10 1/2 inches for four-by-four posts.
Not all fence installers use concrete to set the posts, but the vast majority do. Concrete provides the most strength for your posts. An alternative method is to backfill around the posts with soil and gravel, tamping each layer firmly as you go. This method makes it easier to replace a post in the future if it breaks or splits.
Panel Post Depth
The minimum depth that you should dig your fence post holes for panel sections is 2 feet. A general formula is to dig the holes one-third to one-half of the post's aboveground height. The deeper you dig the holes, the more stability your fence has, but you must also purchase longer posts.
Many builders use 8-foot posts and bury them about 2 feet deep in concrete, leaving a small margin for trimming the tops of the posts to the finished height after the posts and/or the fence panels are installed. Using 10-foot posts allows for deeper burial and leaves more wiggle room for trimming the tops.
Post Hole Depth for Gates
The minimum depth for gate posts is 32 inches. The reason for the greater depth is that the posts must be able to handle the weight and the movement of the gate. Gate posts also should be set in concrete for stability. The movement of the gate will loosen your posts -- and cause them to lean or eventually break -- if you do not anchor them in concrete.
Mixing Your Concrete
Your concrete should be a standard gravel mix. Most homeowners use dry concrete mix sold in bags, but you can also order ready-mix delivered by truck for very large jobs. Bagged concrete commonly comes in 40- and 60-pound bags. If you're using four-by-four posts and have holes that are 10 inches wide by 2 feet deep, you'll need about three and a half 40-pound bags or two and a quarter 60-pound bags for each hole. It's a good idea to overfill the holes slightly and smooth the top of the concrete into a dome that helps shed water away from the post.