In the United States, a standard beer barrel holds 31 U.S. gallons of the mighty brew. That's equal to 248 pints or 3,868 ounces. A keg holds no more than half of a barrel's worth of the sudsy stuff, or 15.5 gallons. Title 27 of the U.S. Code of Federal Regulations specifies the volume of a standard beer barrel for purposes of figuring alcohol tax. The allowed subdivisions of a beer barrel are halves, thirds, quarters, sixths and eighths. Kegs of these volumes, as well as a handy 5-gallon size, can be used to draw beer from barrels.

According to the Brewers' Association, brewers sold 196,241,331 barrels of beer in the U.S. in 2013, worth approximately $100 billion. That equals more than six billion gallons of beer, ale, porter and related brews. Craft beer from small breweries accounted for 15,302,838 of these barrels, a 7.8% market share. Should you encounter a beer barrel in the U.K., you might notice that it's larger than the U.S. version, holding 43.234 U.S. gallons or 36 imperial gallons.