Firkin (Dan. fire, four), the fourth part of a barrel, an old English measure of capacity, variously given as containing from 7 1/2 imperial gallons to 10.987 standard gallons. For ale its capacity was one gallon less than for beer; as usually reckoned, it was 8 gallons for the former and 9 for the latter. In the United States, firkin designates a tub, usually of butter, the weight of which should be 56 lbs. In some parts, of Pennsylvania it is 110 lbs.