Alcohol proof is a measure of the content of ethanol (alcohol) in an alcoholic beverage. The term was originally used in England and was equal to about 1.821 times the alcohol by volume (ABV). The UK now uses the ABV standard instead of alcohol proof. In the United States, alcohol proof is defined as twice the percentage of ABV.
The measurement of alcohol content and the statement of content on bottles of alcoholic beverages is regulated by law in many countries.
The term proof dates back to 16th century England, when spirits were taxed at different rates depending on their alcohol content. Spirits were tested by soaking a pellet of gunpowder in them. If the gunpowder could still burn, the spirits were rated above proof and taxed at a higher rate. As gunpowder would not burn if soaked in rum that contained less than 57.15% ABV, rum that contained this percentage of alcohol was defined as having 100 degrees proof. The gunpowder test was officially replaced by a specific gravity test in 1816.
The value 57.15% is very close to the fraction 4⁄7 ≈ 0.5714. This led to the definition that 100° proof spirit has an ABV of 4⁄7. From this it follows that, to convert the ABV expressed as a percentage to degrees proof, it is only necessary to multiply the ABV by 7⁄4 . Thus pure 100% alcohol will have 100×(7⁄4) = 175° proof, and a spirit containing 40% ABV will have 40×(7⁄4) = 70° proof.
The proof system in the United States was established around 1848 and was based on percent alcohol rather than specific gravity. 50% alcohol by volume was defined as 100 proof. Note that this is different from 50% volume fraction (expressed as a percentage); the latter does not take into account change in volume on mixing, whereas the former does. To make 50% ABV from pure alcohol, one would take 50 parts of alcohol and dilute to 100 parts of solution with water, all the while mixing the solution. To make 50% alcohol by volume fraction, one would take 50 parts alcohol and 50 parts water, measured separately, and then mix them together. The resulting volume will not be 100 parts, but between 96 and 97 parts, since the smaller water molecules can take up some of the space between the larger alcohol molecules (cf. Volume change).
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 a percentage of alcohol by volume (ABV).
The European Union (EU) 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.
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 EU countries, on 1 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 (OIML). This scale, for all intents and purposes the same as the Gay-Lussac scale (GL) previously used by much of mainland Europe, was adopted by all the countries in the European Community in 1980. Using the OIML scale or the GL scale is essentially the same as measuring alcohol by volume except that the figures in the latter case are expressed in degrees, not percentages and measured at a temperature of 15 °C.
In the United States alcohol content is specified as ABV percentage. 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. Proof (the term "degrees proof" is not used), defined as twice the percentage of alcohol by volume, may also be stated. For example, whiskey may be labeled as containing 50% alcohol by volume, and also as 100-proof; 86-proof whiskey contains 43% alcohol.
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.
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