The portion of whisky lost to evaporation during cask maturation is called the angel’s share.
Wooden casks are not completely airtight. As whisky rests inside them, small amounts of alcohol and water pass through the wood and evaporate into the surrounding air. The exact loss varies with the warehouse climate, cask size, wood type, and spirit strength.
Warm, dry conditions can encourage greater evaporation, while cool, damp warehouses may produce a different balance of alcohol and water loss. In Scotland, producers often describe the annual loss as roughly 2% of the cask’s contents, although actual figures vary considerably.
The phrase is a traditional counterpart to “devil’s cut,” a term associated with whisky retained in the wood itself. Angel’s share therefore means evaporation, not leakage, and it does not refer to whisky removed deliberately for tasting or quality control.