Well the formula for the area of a circle is:
Pi X radius squared which in your case would be,
radius 4" or .34 squared = .1156
pi = (3.142)
3.142 X .1156 =.3632
.3636 X 4' of depth of tube = 1.45 cubic feet per tube
1.45 cubic feet per tube X 12 tubes = 17.43 cubic feet.
How many cubic feet are in an 80 lb. bag of sacrete I have no clue, but it should be listed on the bag.
If I were you I would get a few extra bags, it is surpising how much you can loose on the ground when filling those things.
Somebody check my math please.
I have been known to order way too much, and way too little concrete. Just ask the dispacthers.