Alcohol proof is a measure of how much alcohol (ethanol) is contained in an alcoholic beverage. The term was originally used in the United Kingdom and was defined as 7/4 times the alcohol by volume (ABV). The UK now uses the ABV standard instead of alcohol proof. In the United States, alcoholic proof is defined as twice the percentage of ABV.
The measurement of alcohol content and the statement of this content on the bottle labels of alcoholic beverages is regulated by law in the United States. The purpose of the regulation is to provide pertinent information to the consumer.
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From the 18th century until 1 January 1980, the UK measured alcohol content in terms of "proof spirit", which was defined as spirit with a gravity of 12/13 that of water, or 923kg/m3, and equivalent to 57.15% ABV. The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was "proved" by dousing gunpowder with it and then testing to see if the gunpowder would ignite. If it did not, then the rum contained too much water and was considered to be "under proof". Gunpowder would not burn in rum that contained less than approximately 57.15% ABV. Therefore, rum that contained this percentage of alcohol was defined to have "100° (one hundred degrees) proof".
The value 57.15% is very close to the fraction 4/7 = 0.5714. Thus, the definition amounts to declaring that 100° proof spirit has an ABV of 4/7. From this, it follows that to convert the ABV (expressed as a percentage, as is standard, rather than as a fraction) to degrees proof, it is only necessary to multiply by 7/4 = 1.75. Thus pure, 100% alcohol will have 100×(7/4) = 175° proof, and a spirit containing 50% ABV will have 50×(7/4) = 87.5° proof.
The use of "proof" as a measure of alcohol content is now mostly historical. Today, liquor is sold in most locations with labels that state its alcohol content as its percentage of alcohol by volume (ABV).
Many countries also use a measure called a standard drink. In Australia, a standard drink contains 10 g (12.67 ml) of alcohol, the amount that an average adult male can metabolise in one hour. The purpose of the standard drink measure is to help drinkers monitor and control their alcohol intake.
Governmental regulation 
European Union 
The European Union follows recommendations of the International Organization of Legal Metrology (OIML). OIML's International Recommendation No. 22 (1973) provides standards for measuring alcohol strength by volume and by mass. A preference for one method over the other is not stated in the document, but if alcohol strength by volume is used, it must be expressed as a percentage (%) of total volume, and the water/alcohol mixture must have a temperature of 20°C (68°F) when measurement is done. The document does not address alcohol proof or the labeling of bottles.
United Kingdom 
Since 1 January 1980, the United Kingdom has used the ABV standard to measure alcohol content, as prescribed by the European Union.
“In common with other EC countries, on 1st January, 1980, Britain adopted the system of measurement recommended by the International Organisation of Legal Metrology, a body with most major nations among its members. The OIML system measures alcohol strength as a percentage of alcohol by volume at a temperature of 20°C. It replaced the Sikes system of measuring the proof strength of spirits, which had been used in Britain for over 160 years.”
“Britain, which used to use the Sikes scale to display proof, now uses the European scale set down by the International Organization of Legal Metrology (IOLM). This scale, for all intents and purposes the same as the Gay-Lussac scale previously used by much of mainland Europe, was adopted by all the countries in the European Community in 1980. Using the IOLM scale or the Gay-Lussac scale is essentially the same as measuring alcohol by volume except that the figures are expressed in degrees, not percentages.”
United States 
In the United States, alcohol content is measured in terms of the percentage of alcohol by volume. The Code of Federal Regulations (27 CFR [4-1-03 Edition] §5.37 Alcohol content) requires that liquor labels must state the percentage of ABV. The regulation permits, but does not require, a statement of the proof provided that it is printed close to the ABV number. For bottled spirits over 100 ml containing no solids, actual alcohol content is allowed to vary within 0.15% of ABV stated on the label. Alcohol proof in the United States is defined as twice the percentage of alcohol by volume. Consequently, 100-proof whiskey contains 50% alcohol by volume; 86-proof whiskey contains 43% alcohol. Note that in the United States the term "degrees proof" is not used. For example, 50% ABV would be described as "100 proof" rather than "100 degrees proof".
See also 
- Scotch Whisky: Questions and Answers, Section 6. Scotch Whisky Association. http://www.dcs.ed.ac.uk/home/jhb/whisky/swa/chap6.html
- Wine and Health FAQs
- Recommendation No. 22, International Alcoholmetric Tables (PDF). p. 9.
- Regan, Gary (2003). The Joy of Mixology. New York: Clarkson Potter. pp. 356–357. ISBN 0-609-60884-3.
- Title 27 Alcohol, Tobacco and Firearms, Chapter 1, §5.37. Bureau of Alcohol, Tobacco and Firearms; Department of the Treasury. p. 62.