What is the best fence post depth? That depends (of course) on what type of fence you are building and which post you are putting in the ground. We’ll cover a few here with some general fence builder rules.
Privacy Fence Post Depth — 1/3 of Height
The general rule of thumb for privacy fence posts is to bury them 1/3 the depth of the height of the post. This is easy for your typical privacy fence. Use 8? posts, bury 2? in the ground, and you’re left with a 6? post on which to build a fence.
Concrete is still a good idea (I recommend it), especially if you live in an area with high winds or occasional hurricanes. I’ve even drilled a 9/16? x 8? hole in the bottoms of the posts and driven a piece of re-bar in with about 12? sticking out. The result is a post buried 2 feet in the ground, with re-bar another 12? (driven into the clay soil), surrounded by concrete — very strong posts. Overkill? Whatever, dude.
Farm & Ranch Fence
Your typical farm or ranch fence, especially one with wire stretched, has 2 types of posts — line posts and corner posts.
Use the 1/3 of height rule for the line posts depths. With 6 foot posts, you will probably bury 1.5 to 2 feet and end up with a 4 to 4.5 foot post. For corner posts, you might want to use a rule of 1/2 the height for the fence post depth. So, a 5 foot tall corner post would probably be buried 2.5 feet in the ground — probably more like 3 feet buried of an 8? post.