Full answer
The full US liquid measurement hierarchy: 1 gallon = 4 quarts = 8 pints = 16 cups = 32 gills = 128 fluid ounces = 3,785 milliliters. The cleanest way to remember it: each step doubles — 1 cup doubles to a pint, a pint doubles to a quart, and four quarts make a gallon.
Half gallon (common for milk and juice containers) = 8 cups = 64 fl oz. Quarter gallon = 1 quart = 4 cups = 32 fl oz. These come up constantly in cooking and are worth memorizing.
The US gallon and the UK (Imperial) gallon are different. The US gallon is 128 fl oz (3.785 liters). The UK imperial gallon is 160 fl oz (4.546 liters) — about 20% larger. Recipes from British cookbooks sometimes cause confusion because of this difference. Most online recipes assume US measurements unless specified.
In baking and large-batch cooking, gallon measurements come up for beverages, brines, and large stock preparations. 1 gallon of water weighs approximately 8.34 pounds. This is useful for large-batch pasta cooking, pickling brines, and calculating the weight of liquid when planning yields.
The "drink a gallon of water per day" trend: 1 gallon = 128 oz = 16 cups. The commonly cited "8×8 rule" (8 glasses of 8 oz per day) equals 64 oz, or half a gallon. The National Academies recommends about 91 oz (11.4 cups) for women and 125 oz (15.6 cups) for men from all beverages and food combined — slightly under a gallon for men.