US gallons to US cups (gal to cup)

One US gallon is exactly 16 US cups: 4 quarts, each 2 pints, each 2 cups. Every step is a doubling and each is a definition.

Worked through: 1 gal x 16 = 16 cup.

Going the other way, one US cup is 0.0625 US gallons.

For mental arithmetic: Multiply by 16: double it four times.

Sixteen cups, and the reason it rarely helps is that nobody drinks a measuring cup. A cup in this sense is 8 fluid ounces; a typical glass of iced tea or lemonade holds 12 to 16. So a gallon of punch serves 16 measured cups but 8 to 10 glasses, and catering estimates are made in servings rather than in cups for that reason. The quick mental map American schools teach (a gallon holds four quarts, a quart two pints, a pint two cups) is the whole of the US liquid system and gets every one of these conversions without a factor.

Precision is worth matching to the input. A measurement given as 0.25 gal carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.