I got the following information from wiki page http://en.wikipedia.org/wiki/Fluid_ounce
You might need to find out which oz is for your beer.

Fluid ounce

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl.) is a unit of volume in both the imperial and the US customary systems. It is common to refer to the unit simply as an ounce, especially in cases where no confusion with the unit of mass (also called an ounce) is likely to occur.

Imperial fluid ounce
    The imperial fluid ounce is 1?160 of an imperial gallon making it very nearly the volume occupied by one avoirdupois ounce of water[1].

        1 imperial fluid ounce  =  1/160 imperial gallons
        =  1/20 imperial pints
        =  1/5 imperial gills
        =  8 imperial fluid drams
        =  28.4130625 millilitres (exactly)[2] (cc)
        ?  1.733871455 cubic inches
        ?  0.960759940 U.S. fluid ounces

U.S. customary fluid ounce
    The U.S. customary fluid ounce is defined to be 1?128 of a U.S. gallon. This volume of cool, pure water weighs about 1.04 avoirdupois ounces (29.5 g).

            1 U.S. fluid ounce  =  1/128 U.S. gallon
        =  1/16 U.S. pints
        =  1/8 U.S. cup
        =  1/4 U.S. gills
        =  8 U.S. fluid drams
        =  1.80468754 cubic inches (exactly)[3]
        =  29.5735295625 millilitres (exactly)[4] (cc)
        ?  1.040842731 imperial fluid ounces

U.S. food labeling fluid ounce
    U.S. 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.[5] This is not meant to concur with the customary US definition exactly but is a common source of confusion.[citation needed]

                    30 millilitres  ?  1.055852392 imperial fluid ounces[2]
        ?  1.014420681 U.S. customary fluid ounces[3][4]
        ?  1.830712323 cubic inches[4]

Other useful conversions

Given the definitions above, the number of US fluid ounces in one liter is:

    1/29.5735295625 1000 = 33.8140

There are thus about 34 customary US fluid ounces in one liter.

Similarly, there are exactly 33 1/3 US food-labeling fluid ounces in a liter.