how do u calculate cf for fiberglass box?

Diverdown, in SO many cases there are way too many curves in an enclosure to actually calculate.  So filling a 12 x 12 x 12 square box with some time of lightweight media is the first and easiest thing to do.  I havent read this thread in a few years so here it is.... https://www.the12volt.com/installbay/forum_posts.asp~TID~41102~PN~1 

You ask about calculating though.  Lets say you need to make an enclosure that contains 1.25 cu. ft of volume. 

We know 12 in. x 12 in. x 12 in. = 1728 cubic inches (aka...1 cubic foot).  Also 1728 divided by 0.25 is 432 cubic inches...add your 1728 and you have a total of 2160 cubic inches to be 1.25 cu.ft.  Make a mark 3 inches up from the bottom of the box.  When you fill the box full of media you know that you have 1 cu.ft. volume of measuring media...you then empty it into a "semi-fabricated" enclosure...but you need that extra 432 cu.in. of media...so fill your the 12x12x12 box up to the 3 inch mark, thats the additional amount (432 cu.in.) of media you would have to add to total 1.25 cubic ft.  (Make a mark 6 inches high would of coarse be 0.5 cu.ft.  A mark 9 inches from the bottom would be 0.75 cu.ft.....and so on.)

....sorry if you knew all of that, but you asked.  :)