This question doesn't have a straightforward answer because "one fifth" isn't a unit of measurement.

To answer this, we need to know what "one fifth" *of* something is. For example:

* One fifth of a gallon: A gallon is 128 ounces, so one fifth of a gallon is 128/5 = 25.6 ounces.

* One fifth of a pound: A pound is 16 ounces, so one fifth of a pound is 16/5 = 3.2 ounces.

Please provide the unit you'd like to convert "one fifth" of.