Understanding Alcohol Proof: How Is Alcohol Proof Measured?

Alcohol proof is a unit of measurement used to determine the amount of ethanol (ethyl alcohol) in alcoholic beverages. The higher the alcohol proof, the stronger the drink. This system of measuring alcohol content is primarily used in the United States, where alcohol proof is defined as being double the amount of alcohol by volume (ABV). For example, if a whiskey is 50 percent alcohol by volume, it is a 100-proof whiskey.

Measuring alcohol strength using the proof system can be traced back to sixteenth-century England, where liquor was taxed at different rates depending on the amount of alcohol it contained. In order for the government to discern if the liquor should be taxed at a higher rate, an assessment known as the “gunpowder test” was performed. This test consisted of soaking a pellet of gunpowder in the liquor, then attempting to ignite it. If the pellet burned, the liquor was therefore strong enough to meet the higher tax threshold and was thus classified as a “proof” spirit.

Today, alcohol proof is calculated differently depending on the country. Here are three ways it is measured:

• In the United States: U.S. law considers alcohol proof to be twice the ABV percentage. So a liquor containing 60 percent alcohol in the U.S. would be 120 proof.
• In France: The French proof system, the Gay-Lussac scale, was developed by French scientist Joseph-Louis Gay-Lussac in 1824 and uses "degrees GL" as its unit of measurement. France considers alcohol proof to be exactly equal to the ABV percentage. So a liquor in France containing 60% alcohol would be 60 degrees proof (or 60 degrees GL).
• Internationally: Most countries, including the United Kingdom, use the European scale developed by the International Organization of Legal Metrology (OIML). This simple method, also known as the ABV standard, is essentially the same as the Gay-Lussac scale except there's no conversion to proof necessary. So a liquor containing 60 percent alcohol would be labeled as 60 percent ABV.

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