The weight of 55 gallons depends on what liquid is being measured. Gallons are a measurement of volume, not weight. To calculate the weight of a certain volume, it is necessary to know the density of the liquid being measured.
For example, the density of water is 1 kilogram per liter. One gallon is equal to 4 liters, so 55 gallons equals 220 liters. To find the weight of 55 gallons of water, multiply the volume of the water by the water's density: 220 liters of water multiplied by 1 kilogram per liter equals 220 kilograms. In the imperial system, the same amount of weight is about 485 pounds.