The US gallon, which is equal to approximately 3.785 L, is legally defined as 231 cubic inches. A US liquid gallon of water weighs about 8.34 pounds or 3.78 kilograms at 62 °F (17 °C), making it about 16.6% lighter than the imperial gallon.
volume of water = 1 gal= .0037854 cubic meter
density of water= 1000kg per meter cube
weight of water= .0037854 * 1000 * 5 =5 * 3.7854 = 18.927