There are 128 fluid ounces in a gallon.
This unit of volumetric measurement is equivalent to 1/8th of what’s called a wine cask, which back when these were used by the English royal family would hold about 266 gallons of wine.
If you’re looking for liquid measurement for the US’ usage though then there are 161.7825 cubic inches per gallon (approximately).
How does that work out? Well, if you overlook milliliters and just consider centimeters then a gallon has 3785.5185 cm3 volume-wise so I’ll let your math skills do the rest….
Some other units of measure include barrells and hogsheads both measuring 2 barrels or 6 hogs.