barrel of beer (contemporary)

The barrel consumers are most likely to encounter today is the barrel of beer.

United States

The barrel of draft beer holds 31 U.S. gallons. Smaller sizes include half a barrel, 15½ gallons; quarters, also called kegs, 7¾ U.S. gallon; sixths; and eights, also called ponies, of 3.8 U.S. gallons.

William L. Downard.
Dictionary of the History of the American Brewing and Distilling industries.
Westport (CT): Greenwood Press, 1980.

Europe

The beer barrel holds 50 liters.

A correspondent informs us that the English beer barrel still holds 36 imperial gallons, having survived with the pint.