Alcohol by volume (abv) is an indication of how much alcohol (expressed as a percentage of total volume) is included in an alcoholic beverage. This measurement is assumed as the world standard. Another way of specifying the amount of alcohol is alcoholic proof. An "alcoholic proof" is roughly twice the alcohol by volume (although this does vary in some countries). In some countries, alcohol by volume is referred to as degrees Gay-Lussac (after the French chemist Joseph Louis Gay-Lussac). In the United States, many states legally regulate and tax alcoholic beverages according to alcohol by weight(abw) (expressed as a percentage of total mass). In some localities, brewers print the abw (rather than abv) on beverage containers, particularly on low-point versions of popular domestic beer brands. Because alcohol is less dense than the water component of the beverage, alcohol percentage by weight is only 4/5 of the alcohol percentage by volume (e.g., 3.2% abw = 4.0% abv), which has caused confusion among consumers more familiar with the worldwide standard abv.
