Currently DF has several containers for holding liquids but theirs little ability to transfer things between them and no underlying volume system for doing so.
It seems to me their are at least three distinct and separate systems, Barrels, Buckets and Vials. Barrels hold alcohol and are drunk from directly without any interaction with water on the map. Buckets hold only water and are the only means to interact with 'map' water though its unclear exactly how many buckets are needed to fill a 1/7th z level. Vials are used for extracts which can't do anything, but we can assume the vial is smaller then the bucket because the same material that forms one bucket forms 3 vials.
I'd like to see some clearly defined system that states 1/7th z level = X Barrels = Y Buckets = Z vials to that things can be efficiently move between these containers. For example using buckets to fill barrels with water or combining the contents of many vials into a barrel. It need not be a system based on any real world system like cups or pints or letters just so long as its consistent. This will also pave the way for the variety of other liquid based applications so many have asked for like bath tubs or mega barrels.
Such a system should probably be based of the smallest unit of liquid which would most naturally be the 'drink' unit that a dwarf consumes each time they drink from a barrel. If a Vial and water skin was 1-2 'drink' and a Bucket 5-10 that would fit well with Barrels holding 25 drinks. Its all rather flexible because these things clear right now. What do others think the correct numbers really are?
In addition consider how other things will interact with fluids, metal bars can be melted into liquid and this might need to be translated into the liquid system, dose a bar fill a bucket or a vial or neither? Do powdered materials in bags follow a similar system?