Alcohol “Proof” and “Alcohol by Volume”: Definitions and Explanations
In the U.S. 200 proof alcohol (pure alcohol) is rarely found outside a laboratory. That’s because if pure alcohol is exposed to the air, it will absorb moisture from the atmosphere and self-dilute down to about 194 proof.
In the early 1700s, the alcohol content of distilled beverages was determined by using gunpowder. A beverage would be “proved” acceptable by pouring some on gunpowder and lighting it. If it burned steadily with a blue flame, it was considered 100 degrees “proof” and equaled 57.15 ethanol. If it failed to burn, it was considered “underproof” and if it burned too quickly it was “overproof.”
In the United Kingdom, 100 proof equals 57.1% ethanol by volume. The historical origins of this definition can be seen in the gunpowder test.
Mississippi maintained state-wide prohibition of all alcoholic beverages for one-third of a century after National Prohibition was repealed. There are still hundreds of dry or prohibition counties across the United States constituting about 10% of its area.
In the United States, proof is defined as double the percentage of alcohol contained in a solution at 60 degrees Fahrenheit or 15.6 degrees Celsius. Thus, 150 proof would be 75% alcohol and 100 proof would be 50% alcohol.
In France, alcohol content is measured in degrees Gay-Lussac (GL). A hydrometer is used and alcohol strength is expressed as parts of alcohol per 100 parts of the mixture. Thus, a spirit with 40% alcohol by volume equals 40 degrees GL.
Internationally, alcohol strength is measured according to the recommendation of the International Organization of Legal Metrology. The alcohol content of a beverage is measured by distilling off the alcohol, weighing it, and expressing the result as a percentage of alcohol by weight (ABW) or by measuring the volume of alcohol distilled and expressing it as a percentage of alcohol by volume (ABV) at a temperature of 20 degrees Celsius or 68 degrees Fahrenheit. Alcohol by weight (ABW) can be converted to alcohol by volume (ABV) by dividing ABW by .079.
Six states (Colorado, Kansas, Minnesota, Missouri, Oklahoma, and Utah) define beverages of 3.2% ABW or less as “non-intoxicating.” This is a historical relic from the failed experiment of National Prohibition, which existed in the U.S. between January 16, 1920 to December 5, 1933. The 18th Amendment to the U.S. Constitution banned the “manufacture, sale, or transportation of intoxicating liquors” but did not define intoxicating liquors. That was defined in the enabling legislation (the National Prohibition Act, commonly called the Volstead Act) as beverages containing one-half of one percent or more of alcohol by volume.
Although their poof differs, standard drinks of beer, wine and spirits (liquor) contain an equivalent amount of alcohol -- 0.6 ounces each. They’re all the same to a breathalyzer.
As the repeal of Prohibition neared, Congress passed the Cullen-Harrison “non-intoxicating beverage act,” which raised the definition of “non-intoxicating liquors” from 0.5% to 4% ABV (3.2% ABW). This legalized low proof alcoholic beverages unless states specifically prohibited their sale. Consequently, 20 states and the District of Columbia began permitting the sale of low proof alcoholic beverages on April 7, 1933, when the law went into effect.
The 21st Amendment repealed the 18th Amendment and National Prohibition on December 5, 1933, rendering the Cullen-Harrison Act meaningless. However, the 21st Amendment gives states the authority to define and regulate alcoholic beverages within their borders. The six states identified on the right side of this page continue to use the old Cullen-Harrison definition.
- International Center for Alcohol Policies. Lower Alcohol Beverages. Washington: International Center for Alcohol Policies, ICAP Report # 19, April, 2007.
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