Scotch whisky must be bottled at a minimum strength of 40% alcohol by volume under UK regulations.
Distillers may reduce whisky’s strength after maturation by adding water, a practice known as proofing or reduction. A bottle can legally be stronger than 40% ABV, and many cask-strength releases are bottled substantially higher because they receive little or no dilution.
The 40% minimum applies to the whisky as sold, not to its strength during every stage of production. New-make spirit is normally reduced before entering casks, and evaporation during maturation can change the alcohol concentration further.
This rule is separate from the minimum maturation requirement for Scotch whisky. Scotch must mature in oak casks in Scotland for at least three years, but the question of how long it matures and the strength at bottling are different legal requirements. Some producers also use 43% ABV as a common house bottling strength, but it is not the legal minimum.