There are many units:
Liters, cubic centimeters, gallons, teaspoons, tablespoons, fluid ounces, cubic inches, quarts, cups, cubic meters, cubic kilometers, barrels, and many more obscure units.
Fluid ounce is an old-fashioned unit, used to measure volume, or capacity.
Gallon
Liters, milliliters.
Gallons
kg L. l.
Liter
Ancient Greek is the basic unit used to measure liquid. Ancient Greek can change location and epoch.
I assume you mean volume rather than capacity. The milliliter would be the best answer. It is 1 thousandth of a liter.
Megabyte, Gygabyte, and Terabyte.
Milliliter
The Unit Used To Measure A Train Can Be Kilometer.
The liter (or litre) will commonly be used for that.