Four Loko is a line of alcoholic beverages sold by Phusion Projects of Chicago, Illinois, United States. Four Loko's recipe formerly included caffeine. Phusion operates as Drink Four Brewing Company. Four Loko, the company's most popular beverage, debuted in the United States market in 2005 and is available in 49 states, and in 21 countries including Ecuador, Guatemala, Paraguay, The Bahamas, Peru, Mexico, Colombia, Bolivia, Honduras, El Salvador, Nicaragua, Costa Rica, China, Canada and some countries in Europe. The name "Four" is derived from the original drink having four "key ingredients".

Alcohol by volume is a standard measure of how much alcohol is contained in a given volume of an alcoholic beverage. It is defined as the number of millilitres (mL) of pure ethanol present in 100 mL of solution at 20 °C (68 °F). The number of millilitres of pure ethanol is the mass of the ethanol divided by its density at 20 °C (68 °F), which is 0.78945 g/mL. The alc/vol standard is used worldwide. The International Organization of Legal Metrology has tables of density of water–ethanol mixtures at different concentrations and temperatures.