Thanks ken, ~~now I'm pissed at the manufacturer again~~

Now that I see you run the math, I realize I'm the dumb#$%. Half of my diameter is 7.75, not 7.25. And to think I think of myself as a math guy! Plugging that into the formula gives me a volume of roughly 15.42 gallons, so its as advertised. Think with a little ferm cap I will still be good to go as long as I keep a watchful eye on it. Thanks again.