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Grain alcohol or spirits may be labeled using proof rather than percent alcohol. Here's what proof means and an explanation of why it's used and how it's determined.
Alcohol Proof Definition
Alcohol proof is twice the volume percentage of ethyl alcohol (ethanol) in an alcoholic beverage. It is a measure of the ethanol (a specific type of alcohol) content of an alcoholic beverage.
The term originated in the United Kingdom and was defined as 7/4 the alcohol by volume (ABV). However, the UK now uses ABV as the standard to express alcohol concentration, rather than the original definition of proof. In the United States, the modern definition of alcohol proof is twice the percentage of ABV.
Alcohol Proof Example: An alcoholic beverage that is 40% ethyl alcohol by volume is referred to as being '80 proof'. 100-proof whiskey is 50% alcohol by volume. 86-proof whiskey is 43% alcohol by volume. Pure alcohol or absolute alcohol is 200 proof. However, because alcohol and water form an azeotropic mixture, this purity level cannot be obtained using simple distillation.
Since ABV is the basis for calculated alcohol proof, it's useful to know how alcohol by volume is determined. There are two methods: measuring alcohol by volume and measuring alcohol by mass. The mass determination does not depend on temperature, but the more common percent (%) of total volume is temperature dependent. The International Organization of Legal Metrology (OIML) requires volume percent (v/v%) measurements be performed at 20 °C (68 °F). Countries belonging to the European Union may measure ABV using either mass percent or volume percent.
The United States measures alcohol content in terms of alcohol percent by volume. The percentage of alcohol by volume must be labelled, although most liquors also state proof. Alcohol content may vary within 0.15% of ABV stated on the label, for spirits containing no solids and over 100 ml in volume.
Officially, Canada uses US labeling stating percent alcohol by volume, although the UK proof standard may still be seen and heard. Common spirits at 40% ABV are called 70° proof, while 57% ABV is 100 proof. "Over-proof rum" is rum containing greater than 57% ABV or exceeding 100° UK proof.
Older Versions of Proof
The UK used to measure alcohol content using proof spirit. The term came from the 16th century when British sailors were given rations of rum. In order to demonstrate the rum hadn't been watered down, it was "proved" by covering it with gunpowder and igniting it. If the rum didn't burn, it contained too much water and was "under proof", while if it burned, this meant at least 57.17% ABV was present. Rum with this alcohol percentage was defined to be 100° or one hundred degrees proof.
In 1816, the specific gravity test replaced the gunpowder test. Until January 1, 1980, the UK measured alcohol content using proof spirit, which was equivalent to 57.15% ABV and defined to be spirit with a specific gravity 12/13 that of water or 923 kg/m3.
Jensen, William. "The Origin of Alcohol Proof"(PDF). Retrieved November 10, 2015.