How many ounces are in a gallon? There are 128 US Fluid Ounces in a US Gallon. There are 153.722 US Fluid Ounces in an Imperial Gallon. There are 160 Imperial Fluid Ounces in an Imperial Gallon. and there are 133.228 Imperial Fluid Ounces in a US Gallon. If you are asking about the mass measurement ounces, it depends on the density of the fluid. 602 Views · 2 Upvotes · 1 Upvote