Take the dimentions of the enclosure and multiply them...

For example a 4'x2'x2' = 16 cubic feet

Now go to google, and in the search are type:

16 cu ft in gal

Google conversion calculator kicks in and gives you the result:

16 (cu feet) = 119.688311 US gallons

Easy... Also google can convert pretty much any unit into any other.

- Rick