Work out barrel ageing loss instantly with clear inputs, formula shown and shareable results.
Barrel evaporation, the angels' share, compounds geometrically: remaining volume is initial volume times (1 - annual rate) raised to the years. At 3.5% a year, a fifth of the cask is gone after five years and over a third after twelve. Rates depend on warehouse humidity, temperature cycling and cask size.
Angels' share
Remaining volume = initial volume x (1 - annual loss rate)^years
It depends on humidity. In dry warehouses water evaporates faster and strength rises; in humid conditions alcohol is lost preferentially and strength falls. Scottish warehouses typically see strength fall, Kentucky rickhouses see it rise.
Surface area to volume ratio is higher, so both evaporation and wood extraction proceed faster. Small casks mature quickly but lose a much larger proportion of their contents.