How do you find out how many grams are in a gallon?

1 US gallon = 16 US cups

This is true because a cup equals 8 fluid ounces, and a pint equals 16 fluid ounces. Therefore there are 2 cups to a pint, 2 pints to a quart, and 4 quarts to a gallon. Just multiply and you have 2 x 2 x 4 = 16.

You might also know that a gallon equals 128 fluid ounces. 128/8 is also 16.