Quote: The old-timers say 1/3 of the post length if I remember right. Most of mine aren't that deep due to the rock. If the fence is to be permanent, pour dry Sacrete right out of the bag, tamping it down to full depth. and let it set for 48 hours. The Sacrete draws enough moisture from the surrounding ground to set up, saving the hassle of mixing up batches of wet concrete. The posts aren't going anywhere. I have posts that I set 20 years ago like that that are still as tight as the day I set them.