Question 465280
If 8 inches is the diameter of the 8" x 48" cylinders, the volume of each is 
{{{V = pi*r^2*h = 768*pi}}} cubic inches
12V = 4608*pi cu inches =~ 14477 cubic inches.
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The volume of the post is 4*4*48 cu inches = 768 cu inches
12*768 = 9216 cubic inches
If you're putting 12 posts in 12 holes, the volume of cement for all 12 is
14477 - 9216 = 5261 cubic inches.
1 cubic yard = 36*36*36 cubic inches = 46656 cu in
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5261/46656 =~ 0.113 cubic yards