What Does "Angel's Share" Mean?

The "angel's share" is the whisky lost to evaporation through the wood of the cask during maturation.

Oak barrels aren't airtight — they're slightly porous, which is actually essential to how whisky matures (it allows a slow exchange of air in and out of the cask). One side effect of that porosity is evaporation: a small percentage of the spirit in every cask is lost to the atmosphere each year. Distillers have long called this loss the "angel's share" — the poetic idea that the departing spirit is a toll paid to the angels for watching over the warehouse.

How much is lost

The rate of evaporation depends heavily on climate — warmer, drier, more variable conditions generally increase evaporation, while cooler, more stable, humid conditions slow it down. This is one reason ageing the same spirit in different climates (say, a hot Kentucky rickhouse versus a cool Scottish warehouse) can produce quite different results, both in how much whisky is left after a given number of years and in how the remaining spirit tastes.

Why it matters commercially and for flavour

Every litre lost to evaporation is a litre a distillery can't sell, which is part of why very long-aged whisky is expensive — a cask that started at 200 litres might contain significantly less than that after 15–20 years of maturation. Evaporation also concentrates the remaining spirit and its extracted flavour compounds over time, contributing to why very old whisky often tastes richer and more intense.