A gallon of water weighs approximately 8.34 pounds or 3.78 kilograms. This is because water has a density of approximately 1 gram per milliliter (mL) or 1 pound per gallon. So, a gallon of water, which is equal to 128 fluid ounces or 3,785 mL, would weigh around 8.34 pounds or 3.78 kilograms.
It’s important to note that the weight of a gallon of water can vary slightly based on temperature and pressure. Water is densest at a temperature of 39.2 degrees Fahrenheit (4 degrees Celsius) and at atmospheric pressure of sea level. At other temperatures and pressures, the weight of a gallon of water may be slightly different.