According to the National Institute on Alcohol Abuse and Alcoholism, a fifth of spirits contains 750 ml or around 3.2 cups. The hard liquor typically sold in fifths is 40 percent alcohol by volume, or 80 proof.
The NIAA states that a fifth of hard liquor is equivalent to 17 shots. One shot (1.5 fluid ounces) of hard liquor contains around 0.6 fluid ounces of pure alcohol, which is the same amount of alcohol found in 12 fluid ounces of regular beer. According to The Drinks Business, popular spirits sold in fifths include Smirnoff vodka, Bacardi rum, Johnnie Walker whisky, Jack Daniels whisky and Jose Cuervo tequila.