A half gallon of milk contains 8 cups. A gallon is a unit of volume commonly used to measure liquids, and it is equivalent to 128 fluid ounces or 3.785 liters. A cup, on the other hand, is a smaller unit of volume typically used for recipes and everyday measurements, and it is equal to 8 fluid ounces or 0.237 liters. To convert a half gallon to cups, simply multiply the volume in gallons (0.5) by the number of cups in a gallon (8): 0.5 gallons x 8 cups/gallon = 8 cups.