1 Gallon [Fluid, US] = 16 Cups [US]
1 Gallon [Dry, US] = 18.618355 Cups [US]
1 Gallon [UK] = 18.18436 Cups [Metric]
There are 16 cups in a gallon, because from the largest to the smallest units from gallon to cup, there are 4 quarts in a gallon, 2 pints in a quart and 2 cups in each pint, that makes 4 * 2 * 2 = 16 cups in a gallon.
Gallon is an imperial and US customary volume measurement unit. 1 US fluid gallon is 3.785 liters, 1 US dry gallon is 4.4 liters and 1 imperial (UK) gallon is 4.546 liters. The abbreviation for gallon is "gal".
Cup is a volume unit. The imperial and the Canadian cups are no longer in use and replaced by metric cups (250 mL). There are 16 cups in a US liquid gallon and about 18.5 cups in a US dry gallon. There are 18.2 metric cups and 16 imperial cups (284 mL) in an imperial gallon (4546 mL). The abbreviation for cup is "c".