That's a lot easier if you convert to metric. Volume = height in mm x area of base (3.14 x radius in mm squared) and 1000 cubic millimeters = 1000 milliliters so once you get the volume in mm^3 it's the same as the liquid volume in ml. Then just convert your ml back to fl oz. There's a nice little metric/imperial converter at http://www.sciencemadesimple.com/conversions.html

Matt