We're going to build up a 4' fence around our property sometime in here in the spring or summer with 4x4 posts.
I've seen that you should bury 1/3 to 1/2 of the above ground post height; so this would make our hole 1.5-2' deep with gravel below. I've also seen that the diameter of the hole for a 4x4 post should be 12".. which would make the radius 6". After going through the math for the hole ( pi x r^2 x h ) it comes out that the hole will have a volume of 1.57 cu ft, but the post itself will take up 0.17 cu ft, so the volume to be filled with concrete will become 1.4 cu ft per post. Right?
The question I have now is that a 50lb bag of fast-setting Quickrete apparently fills 0.375 cu ft after setting. This would mean that we'd need almost 4 bags per post (3.75) - this is also the number that comes up from Quickrete's online calculator. I'm not against this exactly, except that I've seen people saying they've used half a bag per post or a bag a post. How accurate is this and how much should we need? At that rate, we're almost looking at 100+ bags. The 80lb bags fill about 0.6 cu ft and are about a dollar cheaper, is there anything against going with it?
What type of concrete do I need to use, and how much?