What is "one drink?"
Knowing how to count a standard drink is necessary for calculating blood alcohol concentrations. Too often, people underestimate how much they have had to drink because they aren't using standard measurements.
Beer
One drink = one 12-ounce beer. This is normal-strength beer (5% alcohol).
Malt liquor ranges from 6-9% alcohol, so 12 ounces of malt liquor is approximately 1.5 drinks; 40 ounces of malt liquor is 4.5 drinks.
Liquor
One drink = 1.5 ounces of liquor (40% alcohol or 80 proof). This is how much whiskey, vodka, gin, tequila, brandy, cognac, etc. is in a measured mixed drink or in a standard-size shot glass. Remember that mixed drinks may not be measured and often contain far more than 1.5 ounces of alcohol.
Grain alcohol (Everclear) is 95% alcohol or 190 proof and some rums like Bacardi 151 are 151 proof or 75% alcohol. These liquors are banned in many states because of their high alcohol content.
Wine
One drink = 5 ounces of standard wine (12% alcohol). This is most table wines: white, red, rosé, champagne.
One drink = 3-4 ounces of fortified wine (17% alcohol). This is wine with 13% or more alcohol content, such as sherry or port.