I doubt I will hurt the OP's feelings (the post is over 3 years old) when I wonder how a guy that does fencing has to start a thread on how asking much concrete he needs for his pour. (hint; read the bag. If you can't compute the cubic feet required, get a job in fast food)
One thing that is worth mentioning in this thread, or repeating;
The concrete posted in post #53 shows 2 different formulations. The "contractor mix" at Menards costs about the same but is much stronger, however, comes in 80# bags. The homeowner, "cr@p" grade is anemic, weak stuff that barely looks like it has any Portland in it. It's cheaper and more user friendly because it is in 60# bags, but essentially the cost per serving is about the same. This is a great example where you really don't get what you pay for. Always buy the better grade.
Also....to repeat..... buy enough to do the job. The expense in in your time, not the materials used.
For abetter quality job use enough material, dig deep enough, if needed flare the base, pour slightly proud of the surface and if possible slightly convex to move water away from the posts.
That is all.
Willy