Cask strength (also known as barrel proof) is a term used in whisky-making to describe the level of alcohol-by-volume (abv) strength that is used for a whisky during its storage in a cask for maturation – typically in the range of 60–65% abv.

Most bottled whisky is diluted with water to bring its strength (i.e., its abv level) down to a level that makes it less expensive to produce and more palatable to most consumers, usually about 40% abv – a level that is the statutory minimum in some countries. The degree of dilution is said to bring out different flavours of the whisky, which can affect the decisions of the producers regarding the concentration they select for the bottling of their various brands.

While the majority of whisky bottled by distillers is watered down to about 40% abv, some whiskies are bottled at cask strength.

These whiskies are usually produced for the upper premium market category, as cask strength whisky is more expensive to produce and less palatable to casual consumers than diluted whisky. Such whiskies are usually older as well, as the whisky develops flavors coming from the long marriage of cask and liquor, to the extent that substantial aging is demanded by the more sophisticated premium-market consumers.