Sure thing. It's π•r^2•L
(3.14•1/2 the diameter squared• length of exposed)
That gives you the area of the tube in inches. Then double it for there being 2 tubes. Then divide that by 1728 which is a cubic foot in inches.
Edit cause that didn't work.
3.14 x radius squared x length x number of tubes. Divide that by 1728 (12x12x12) gives cubic feet