How much hard alcohol is considered one drink?

According to the National Institute on Alcohol Abuse and Alcoholism (NIAAA), one drink of hard alcohol is equivalent to:

- 1.5 fluid ounces (44 milliliters) of 80-proof distilled spirits such as vodka, whiskey, gin, and rum

- 1 fluid ounce (30 milliliters) of 100-proof distilled spirits

- 0.75 fluid ounces (22 milliliters) of 120-proof distilled spirits