The 2.5g is the actual volume of water currently in there. I might be able to bump it up to 3g if I can adjust things a bit more (I figure the total volume of the pot is about 4g minus all the stuff and filled to the brim). There's a pretty good amount of swimming room in there, too. That big rock in the front is flat and less than 1" thick, so it doesn't take up as much space as it looks like it would.