Ounces and Fluid Ounces vary.
A floz is 29.57 ml or grams. This varies by country and system.
"US regulation 21 CFR 101.9(b)(5)(viii) also defines a fluid ounce as exactly 30 millilitres, but this is for use in nutrition labeling only" (Wikopedia)
Anyway, ounces, fluid ounces, Imperial gallons, gallons, is all screwwed up. I usually go with ~28g=1ounce unless I get rounding errors for larger numbers (over 3 ounces.)
A fl ounce of alcohol does NOT weigh 1 ounce. Closer to .78ounce by density. It has some water in it so I allow for that by rounding to .8. Not real accurate, I am backpacking...