A keg's volume varies based on the type, but the most common keg in the U.S. is the half-barrel keg, which holds 15.5 gallons of liquid. Other typical keg sizes in the U.S. include:

• Half-barrel keg: 15.5 gallons
• Quarter-barrel keg (pony keg): 7.75 gallons
• Sixth-barrel keg (sixtel): 5.16 gallons
• Cornelius keg (homebrew keg): 5 gallons

Kegs are generally smaller than barrels (which are 31 gallons in the U.S.) and sizes can vary by country and brewery. For example, in Europe, the standard keg size is often 50 liters (approximately 13.2 gallons).