How deep should a fence post be with respect to the bottom of the concrete base?

Question

Seems like a simple question, but I have been searching for a while. Everything I find is about the depth of the hole for the concrete, not the depth of the post.
I want to know how far a post must go into a concrete hole. The concrete is well set, but has some muck in it. I need to know how deep does the post go, so that I know how much muck to clear.
Posts are wood. Post/hole width varies 2→3 inches (don't ask). Fence height will be about 4 feet.
So how much post needs to be in the concrete, so that the concrete can hold the post?

FreeMan · Accepted Answer

Welcome back!
If I understand correctly, you have concrete in the ground already and there's a hole in the middle of the concrete and you want to put the post into the hole. The general rule is that 1/3 to 1/2 of the post's height above ground should be below ground, so if your post will show 48" above ground then you should have

48" / 3 = 16"

below ground at a minimum, up to

48" / 2 = 24"

would probably be better.
While I don't see any inherent reason why it won't work, it does seem odd to be putting posts into precast concrete plugs in the ground. It's also a bit odd that the holes in the concrete are 2" - 3" around. As noted in one of the comments, I'm not sure what kind of fence post you'll be putting into this hole. Will you be trimming down a 4x4" post?
You may consider using some sort of steel post to lever these chunks of concrete out of the ground and pouring new concrete in place around your new posts.

Evil Elf · Answer

I think the Fence Bible described a method for putting a few inches of gravel at the bottom of the hole, resting the post on that for drainage, then pouring concrete.

How deep should a fence post be with respect to the bottom of the concrete base?

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