The minimum alcohol strength for Scotch whisky at bottling is 40% ABV. This rule is set by the Scotch Whisky Regulations, which govern how Scotch may be produced, matured, labelled and sold.
Scotch whisky must mature in Scotland for at least three years, but age alone does not determine its legal bottling strength. Producers commonly dilute whisky with water after maturation to reach a chosen strength. Many standard blends are bottled at 40% ABV, while cask-strength releases are usually considerably stronger.
The abbreviation ABV means alcohol by volume. It is different from proof systems used in some countries, especially the United States. A bottle labelled 40% ABV contains 40 millilitres of pure alcohol per 100 millilitres of liquid.
The 40% threshold is a legal minimum, not a statement that every Scotch tastes the same at that strength. Single malts, blended malts and blended Scotch can all have very different flavours, even when bottled at the same alcohol percentage.