Approximately 7771 pennies will fit in a gallon jug. There is a volume of approximately 231 cubic inches in a gallon jug and each penny has a volume of approximately 0.029727 cubic inches. The answer can be deduced from dividing the former with the latter.
The amount of pennies that can fit in a gallon jug may vary depending on the shape of the gallon jug. If the sides of the gallon jug are shaped to fit the pennies, then it is possible for more coins to be placed within the jug. If the sides of the gallon jug are not shaped to fit the pennies, however, then less pennies would fit in the gallon jug.