The amount of gallons of beer in a keg depend on the keg size. The standard size keg is called a full-size keg, which holds 15.5 gallons of beer.
The largest size keg is the standard, but there are many other sizes with various gallons of beer in them. The smallest is a mini keg with 1.32 gallons. Other sizes of kegs include the home brew with 5 gallons, sixth barrel with 5.6 gallons, quarter barrel with 7.75 gallons and the slim quarter, which also has 7.75 gallons. These smaller kegs are better for smaller gatherings or parties with fewer people.