A standard 40% ABV whisky contains 40 ml of pure alcohol per 100 ml of liquid. A typical 25 ml pour equals 10 ml of pure alcohol.
27 CFR Part 5 -- Labeling and Advertising of Distilled Spirits alcohol content in whisky
In the United States, proof is exactly double the ABV (e.g., 40% ABV is 80 proof). In the historical British system, "100 proof" was roughly 57.06% ABV, though this was largely replaced by ABV in 1980. Evolution of Alcohol Strength During Production A standard 40% ABV whisky contains 40 ml