Beer Geek 101 - Keg Volumes

U.S. keg sizes Most U.S. brewers sell beer in 1/2 barrels between 14 and 16 gallons, 1/4 barrels between 7 and 8 gallons, and 1/6 barrels between 5 and 6 gallons.
Since keg sizes are not standardized, the keg cannot be used as a standard unit of measure for liquid volumes. This size standard varies from country to country and brewery to brewery with many countries using the metric system rather than U.S. gallons.

A typical keg (half-barrel) with single opening in the center of the top end

A full keg is a 15.5 U.S. gallon barrel, routinely called a half-barrel. A Quarter Barrel has a volume of 7.75 U.S. gallons. Generally a keg is a vessel smaller than a barrel; thus, it is 30 gallons or smaller.
In the U.S. the terms half-barrel and quarter-barrel are derived from the fact that a U.S. beer barrel is legally defined as being equal to 31 U.S. gallons (note that this is not the same volume as some other units commonly known as barrels). A 15.5 U.S. gallon keg is also equal to:

• 12.7 Imperial gallons
• 58.67 liters
• 103.25 Imperial pints
• 124 U.S. pints
• 165 twelve fluid ounce (U.S. measure) drinks
• About 90 bombers
• About 6.88 24-unit cases of 12 fl oz cans.
• About 5.5 30-racks of 12 fl oz cans.
• 1984 fluid ounces (U.S.)

However, beer kegs can come in many sizes, the typical home brewer uses a "Corny Keg." typically these are old converter soda syrup kegs and hold 5 gallons.