The US fluid ounce equals to about 29.6 milliliters, and the imperial fluid ounce equals to about 28.4 milliliters. Currently, there are two main definitions of the gallon: the imperial gallon (approximately 4.54 liters) and the US liquid gallon (approximately 3.78 liters). There is also the US dry gallon (approximately 4.4 liters), but it is a rare unit. And the number of ounces in a gallon is different in the imperial and the US systems: 1  One US gallon has 128 US fluid ounces.  One imperial gallon has 160 imperial fluid ounces.