The alcohol content of mixed drinks varies greatly, and the potency of a specific drink can vary according to the recipe and the person making it.
It's easy to determine the alcohol content by volume (ABV) of a drink if you know the ingredients and the amount of each. It's simply a matter of calculating the amount of pure alcohol in each ingredient, summing these and dividing by the total volume of liquid.
Multiply the amount of each spirit by its percentage of alcohol. The ABV of any liquor is listed on the label, as this is required by law. For example, to mix a Harvey Wallbanger, you might use 1.25 ounces vodka, 3 ounces orange juice and 0.5 ounce Galliano. Assume the labels show that the vodka contains 40 percent ABV and the Galliano contains 30 percent ABV. For the vodka, multiply 1.25 ounces x 40 percent = 0.5 oz of alcohol (40 percent is expressed in decimal form as 0.40). Similarly, for Galliano, multiply 0.5 ounce x 30 percent = 0.15 ounce of alcohol. Write the amounts of alcohol on a piece of paper.
Sum the amounts of alcohol. In the example, 0.5 + 0.15 = 0.65 ounce of alcohol.
Add the total volume of each ingredient in the drink. Include both alcoholic and non-alcoholic ingredients. In the example, 1.25 ounces vodka + 3 ounces orange juice + 0.5 ounce Galliano = 4.75 ounces.
Divide the total alcohol by the total volume of liquid. In the example, divide 0.65 ounce alcohol by 4.75 ounces total liquid. This equals 0.136. Move the decimal two places to the right to express this as a percentage. The drink in the example contains 13.6 percent ABV.