1 Gallon [Fluid, US] = 3.7854118 Liters
1 Gallon [Dry, US] = 4.4048838 Liters
1 Gallon [UK] = 4.54609 Liters
There are 3.7854118 liters in a gallon, because by definition one gallon is 231 cubic inches and one liter equals to the volume of one kilogram of water (1000 mL or 61.0237 cubic inches), that makes 231 / 61.0237 = 3.7854118 liters in a gallon.
For example, to find out how many liters of water or milk in a half gallon, multiply the gallon value by the conversion factor, that makes 1/2 * 3.7854118 = 1.89 liters of water in a half gallon.
1 US fluid gallon of water weighs about 8.34 lbs or 133.44 ounces or 3.785 kilograms (kilograms equals to 1 liter of water). 1 US fluid gallon of milk weighs about 8.6 pounds (it can be a bit lighter or heavier based on the fat percentage), or 137.6 ounces or 3.9 kilograms.
1 UK gallon of water weighs 10 lbs or 160 ounces or about 4.54 kilograms. 1 UK gallon of milk weighs about 10.3 pounds (it can be a bit lighter or heavier based on the fat percentage), or 164.8 ounces or 4.67 kilograms.
Gallon is an imperial and US customary volume unit and equals to 3.785 liters. There are 4 quarts, 8 pints, 16 cups, 128 fl. ounces, 256 tablespoons or 768 teaspoons in a gallon. The abbreviation is "gal".
Liter (litre) is a metric volume unit and equals to 0.26 US gallons and 0.22 imperial gallons. The abbreviation is "L".