How do I calculate the concrete needed for a fence post?
Well, I always use fast setting fence post concrete and you put 2" of gravel on the bottom of the hole, then fill the hole 1/3 of the way up with water, dump one bag of fence post concrete and level the post, then hold it that way for 2 minutes and let dry overnight. The reason fence post concrete is different is that it is packaged to be one bag per post based on a 12-18" hole diameter and a 4x4 post, no pre-mixing, fast setting.