Alcohol Proof and Alcohol by Volume: Definitions and Explanations

Origins of Alcohol Proof

What’s alcohol proof? In the early 1700s, people measured the alcohol content of distilled beverages by using gunpowsder. That is, they would “prove” the beverage acceptable. To do so they poured some on gunpowder and lit it. If it burned steadily with a blue flame, it was 100 degrees “proof” and equaled 57.15 ethanol. If it did not burn, it was “underproof.” And if it burned too quickly it was “overproof.”


Pure alcohol (200 proof alcohol) is rarely exists outside a laboratory. That’s because if pure alcohol is exposed to the air, it will absorb moisture from the atmosphere. Thus it will and self-dilute down to about 194 proof. 

United Kingdom

In the United Kingdom, 100 alcohol proof equals 57.1% ethanol by volume. The gunpowder test is the origin of this definition of proof.

United States

In the United States, alcohol proof is double the percentage of alcohol contained in a solution. The solution is measured at 60 degrees Fahrenheit or 15.6 degrees Celsius. Thus, 150 proof would be 75% alcohol and 100 proof would be 50% alcohol.

Mississippi had state-wide prohibition of all alcoholic beverages for one-third of a century after the repeal of National Prohibition. Hundreds of dry or prohibition counties exist across the United States. They make up about 10% of its area. 


In France, alcohol content is in degrees Gay-Lussac (GL). A technician tests the solution with a hydrometer. Then the person expresses alcohol strength as parts of alcohol per 100 parts of the mixture. Thus, a spirit with 40% alcohol by volume equals 40 degrees GL.


Internationally, the International Organization of Legal Metrology recommends how to measure alcohol strength. A technician distills away all the alcohol, condenses it, and weighs it. The person then expresses the results a percentage of alcohol by weight (ABW).

Alternatively, the technician measures the volume of alcohol distilled and expresses it as a percentage of alcohol by volume (ABV). The person performs the measurement at 20 degrees Celsius or 68 degrees Fahrenheit. Dividing ABW by .079 converts it to ABV.

Alcohol Equivalence

alcohol proofThe alcohol proof of standard drinks of beer, wine and spirits (liquor) differ. Yet each contain an equivalent amount of alcohol. That is, 0.6 ounce each. They’re all the same to a breathalyzer.

Congress passed the Cullen-Harrison “non-intoxicating beverage act” as the repeal of Prohibition neared. The act raised “non-intoxicating liquors” from 0.5% to 4% ABV (3.2% ABW). This legalized low alcohol proof beverages. However,  states could specifically maintain their own prohibition. Consequently, 20 states and the District of Columbia began permitting the sale of low proof alcoholic beverages. They did so on April 7, 1933, when the law became effective.

In six states beverages of 3.2% ABW or less are “non-intoxicating.” Those states are Colorado, Kansas, Minnesota, Missouri, Oklahoma, and Utah. This is a historical relic from National Prohibition ( January 16, 1920 to December 5, 1933). The 18th Amendment banned the “manufacture, sale, or transportation of intoxicating liquors.” But what, exactly, were intoxicating liquors? That was defined in the enabling legislation. The National Prohibition Act (the Volstead Act) answered that question. Specifically, it was any beverage with one-half of one percent or more of alcohol by volume.

The 18th Amendment repealed the 21st Amendment and National Prohibition on December 5, 1933.  Of course, this made 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 Proof Beverages. Washington: The Center, ICAP Rept #19.


See Also

Alcohol Equivalence

Alcohol Trivia