One US liquid gallon of fresh water weighs roughly 8.34 pounds (lb) or 3.785 kilograms (kg) at room temperature. The weight of water will always depend on its temperature. There are two different ways to measure a gallon in the US: dry or liquid. A US liquid gallon is defined as a volume of 231 cubic inches or 3.785 liters (L). The density (mass divided by volume) of water is 1 kg per L and 1 L is equal to 1 kg. We know that 1 kg is approximately 2.2 lbs. Therefore, one US liquid gallon of fresh water weighs roughly 8.34 lbs. or 3.785 kg.