The pro mix bale is compressed quite a bit so there is actually more than 3.8 cf of soil in there. Closer to 7 according to the pro mix site that says it decompresses to nearly double the size. You might get closer to 8-10 depending on if the pot is actually 5 gal and how tight you pack it in there. The bale I bought last year, I filled something like 2-5 gal, 4-3 gal, 12-1 gal, 2 seed starting trays and several 3.5" pots. There is more in there than you think.
jacob