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 distilled beverages (also called liquors or spirits) 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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In the 18th century and until 1 January 1980, the United Kingdom defined alcohol content in terms of “proof spirit”, which was defined as the most dilute spirit that would sustain combustion of gunpowder.[1] 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 in it, then tested 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”. It was found that gunpowder would not burn in rum that contained less than 57.15% abv. Therefore, rum that contained this percentage of alcohol was defined to have "100 degrees proof".
An alcohol content of 57.15% abv is very close to a 4:7 ratio of alcohol to the total volume of the liquid. Thus, the definition amounted to declaring that (4÷7) × 175 = 100 degrees proof spirit. From this it followed that pure, 100% alcohol had (7÷7) × 175 = 175 degrees proof spirit, and that rum containing 50% abv had (3.5÷7) × 175 = 87.5 degrees proof spirit. To convert the percentage of abv to degrees proof spirit, it was only necessary to multiply the percentage by 1.75.
The use of "proof" as a measure of alcohol content is now mostly historical. Today, liquor is sold with labels that state its alcohol content as its percentage of alcohol by volume (abv). United States law requires that liquor labels must state the percentage of alcohol by volume. The proof number may also be placed on the label, provided that it is close to the abv number.[2]
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.[3] The purpose of the standard drink measure is to help drinkers monitor and control their alcohol intake.
From the 1740s until 1816, Customs and Excise and London brewers and distillers used Clarke’s hydrometer to measure degrees proof. Under the Hydrometer Act of 1818, the Sikes hydrometer was used to measure proof; it remained in use until 1980. The Customs and Excise Act of 1952 defined “spirits of proof strength” (i.e., proof spirits):
“Spirits shall be deemed to be at proof if the volume of the ethyl alcohol contained therein made up to the volume of the spirits with distilled water has a weight equal to that of twelve-thirteenths of a volume of distilled water equal to the volume of the spirits, the volume of each liquid being computed as at fifty-one degrees Fahrenheit.”[1]
The European Union follows recommendations of the International Organization of Legal Metrology (OIML). OIML's International Recommendation No. 22 (1973)[4] 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 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 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.”[1]
“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.”[5]
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.[2] 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.[1] 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.[1] The terminology used in the United States is "n" proof, where "n" is a number — not "n" degrees proof. The term "degrees proof" is not used.
In beer, alcohol content is 3–12% abv, but it is most often 4–6%. Conventional wines will have 9–16% abv, with most being between 12.5–14.5%.[6] Fortified wines, such as port, madeira, marsala, sherry, and vermouth, have 15.5–20% abv. The wine is “fortified” by adding a distilled spirit, usually brandy, to it. Stronger drinks, such as liquors, are distilled after fermentation to increase their alcohol content. These form a very broad category of beverages whose alcohol content can range from 20% to 95% abv. Old recipes may reference second proof (60% abv or 120 proof), third proof (70% abv or 140 proof), or fourth proof spirits (80% abv or 160 proof).