What is the percentage of alcohol in an 80-proof bottle of vodka?

The term "proof" is used to measure the alcohol content in distilled spirits like vodka. In the United States, the proof is defined as twice the percentage of alcohol by volume (ABV). Therefore, an 80-proof vodka contains 40% alcohol by volume. It means that 40% of the liquid in the bottle is pure alcohol, while the remaining 60% consists of water and other possible components.