How would a bar do its inventory system for liquor?

[emptywallet]

How would a bar do its inventory system for liquor? Would they do it by just the bottle count? I've heard of some places that do it by weight of the bottle. Or by how many times they can pour the entire contents of the bottle into a single shot glass, since for all drinks they use a stardard measure of a shot glass for that particular liquor portion of the drink.

 

[flex123]

When I worked at a club we just counted the empty bottles at the end of the night.