Alcohol proof (usually termed simply "proof" in relation to a beverage) is a measure of the content of

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

(alcohol) in an

alcoholic beverage Drinks containing alcohol (drug), alcohol are typically divided into three classes—beers, wines, and Distilled beverage, spirits—with alcohol content typically between 3% and 50%. Drinks with less than 0.5% are sometimes considered Non-al ...

. The term was originally used in

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

and from 1816 was equal to about 1.75 times the percentage of

alcohol by volume Alcohol by volume (abbreviated as alc/vol or ABV) is a common measure of the amount of Alcohol (drug), alcohol contained in a given alcoholic beverage. It is defined as the volume the ethanol in the liquid would take if separated from the rest ...

(ABV). The

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

today uses ABV instead of proof. In the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, alcohol proof is defined as twice the percentage of ABV. The definition of proof in terms of ABV varies from country to country. The measurement of alcohol content and the statement of content on bottles of alcoholic beverages is regulated by law in many countries. In 1972,

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

phased out the use of "proof"; in 1973, the

European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...

followed suit; and the United Kingdom, where the concept originated, started using ABV instead in 1980. The

United States Code The United States Code (formally The Code of Laws of the United States of America) is the official Codification (law), codification of the general and permanent Law of the United States#Federal law, federal statutes of the United States. It ...

mandates the use of ABV, but permits proof to be used also. The

degree symbol The degree symbol or degree sign, , is a glyph or symbol that is used, among other things, to represent degrees of arc (e.g. in geographic coordinate systems), hours (in the medical field), degrees of temperature or alcohol proof. The symbo ...

(°) is sometimes used to indicate alcohol proof, either alone (e.g. 10°) or after a space and joined to the letter P as a unit name (e.g. 13 °P).

History

The term ''proof'' dates back to 16th century England, when spirits were taxed at different rates depending on their alcohol content. Similar terminology and methodology spread to other nations as spirit distillation, and

taxation A tax is a mandatory financial charge or levy imposed on an individual or legal person, legal entity by a governmental organization to support government spending and public expenditures collectively or to Pigouvian tax, regulate and reduce nega ...

, became common. In England, spirits were originally tested with a basic "burn-or-no-burn" test, in which an alcohol-containing liquid that would ignite was said to be "above proof", and one which would not was said to be "under proof". A liquid just alcoholic enough to maintain combustion was defined as 100 proof and was the basis for taxation. Because the

flash point The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture". The flash point is somet ...

of alcohol is highly dependent on temperature, 100 proof defined this way ranges from 20% at to 96% at alcohol by (ABW); at 100 proof would be 50% AB. Another early method for testing liquor's alcohol content was the "gunpowder method".

Gunpowder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...

was soaked in a spirit, and if the gunpowder could still burn, the spirit was rated above proof. This test relies on the fact that

potassium nitrate Potassium nitrate is a chemical compound with a sharp, salty, bitter taste and the chemical formula . It is a potassium salt of nitric acid. This salt consists of potassium cations and nitrate anions , and is therefore an alkali metal nit ...

(a chemical in gunpowder) is significantly more soluble in water than in alcohol. While less influenced by temperature than the simpler burn-or-no-burn test, gunpowder tests also lacked true reproducibility. Factors including the grain size of gunpowder and the time it sat in the spirit impact the dissolution of potassium nitrate and therefore what would be defined as 100 proof. However, the gunpowder method is significantly less variable than the burn-or-no-burn method, and 100 proof defined by it is traditionally defined as 57.15% ABV. By the end of the 17th century, England had introduced tests based on

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

for defining proof. However, it was not until 1816 that a legal standard based on

specific density Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

was defined in England. 100 proof was defined as a spirit with the

pure water Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently pur ...

at the same temperature. From the 19th century until 1 January 1980, the UK officially measured alcohol content by proof spirit, defined as spirit with a

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

of that of water, or , and equivalent to 57.15% ABV. The value 57.15% is very close to the fraction . This led to the approximation that 100-proof spirit has an ABV of . 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 . Thus pure 100% alcohol will have 100×() = 175 proof, and a spirit containing 40% ABV will have 40×() = 70 proof. The proof system in the United States was established around 1848 and was based on percent alcohol rather than specific gravity. Fifty percent alcohol by volume was defined as 100 proof. This is different from 50%

volume fraction In chemistry and fluid mechanics, the volume fraction \varphi_i is defined as the volume of a constituent ''V'i'' divided by the volume of all constituents of the mixture ''V'' prior to mixing: :\varphi_i = \frac . Being dimensionless quantit ...

(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 (see volume change). The use of proof as a measure of alcohol content is now mostly linguistic and historical. Today,

liquor Liquor ( , sometimes hard liquor), spirits, distilled spirits, or spiritous liquor are alcoholic drinks produced by the distillation of grains, fruits, vegetables, or sugar that have already gone through ethanol fermentation, alcoholic ferm ...

is sold in most locations with labels that state its percentage alcohol by volume.

Governmental regulation

European Union

The

(EU) follows recommendations of the

International Organization of Legal Metrology The International Organization of Legal Metrology ( - OIML), is an intergovernmental organisation that was created in 1955 to promote the global harmonisation of the legal metrology procedures that underpin and facilitate international trade. Such ...

(OIML). OIML's International Recommendation No. 22 (1973) provides standards for measuring alcohol strength by

volume Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...

and by

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

. 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 at a temperature of . The document does not address alcohol proof or the labeling of bottles.

United Kingdom

On 1 January 1980, Britain adopted the ABV system of measurement prescribed by the European Union, of which it was then a member. The OIML recommendation for ABV used by the EU states the alcohol by volume in a mixture containing alcohol as a percentage of the total volume of the mixture at a temperature of . It replaced the Sikes

hydrometer A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically Calibration, calibrated and Graduation (instrument), graduated with one or more scales suc ...

method of measuring the proof of spirits, which had been used in Britain for over 160 years.

United States

In the United States, alcohol content is legally mandated to be specified as an ABV percentage. For bottled spirits over containing no solids, actual alcohol content is allowed to vary by up to 0.15% of the ABV stated on the label. By contrast, bottled spirits which are less than 100 ml (as well as those which otherwise contain solids) may vary by up to 0.25%. Proof (the term ''degrees proof'' is not used), defined as being twice the percentage of alcohol by volume, may be optionally stated in conjunction with the ABV. For example, whisky may be labeled as 50% ABV and as 100 proof; 86-proof

whisky Whisky or whiskey is a type of liquor made from Fermentation in food processing, fermented grain mashing, mash. Various grains (which may be Malting, malted) are used for different varieties, including barley, Maize, corn, rye, and wheat. Whisky ...

contains 43% ABV. The most typical bottling proof for spirits in the United States is 80 US proof, and there is special legal recognition of 100-proof spirits in the

bottled in bond Bottled in bond (BIB) is a label for an American-produced distilled beverage that has been aged and bottled according to a set of legal regulations contained in the United States government's Standards of Identity for Distilled Spirits, as origin ...

category defined since 1897. The

Code of Federal Regulations In the law of the United States, the ''Code of Federal Regulations'' (''CFR'') is the codification of the general and permanent regulatory law, regulations promulgated by the executive departments and agencies of the federal government of the ...

requires that liquor labels state the percentage of ABV at a temperature of . The regulation permits, but does not require, a statement of the proof, provided that it is printed close to the ABV number. In practice, proof levels continue to be stated on nearly all spirits labels in the United States, and are more commonly used than ABV when describing spirits in journalism and informal settings.

Canada

Beverages were labelled by alcohol proof in Canada until 1972, then replaced by ABV.

References

{{Reflist

proof Proof most often refers to: * Proof (truth), argument or sufficient evidence for the truth of a proposition * Alcohol proof, a measure of an alcoholic drink's strength Proof may also refer to: Mathematics and formal logic * Formal proof, a co ...

Customary units of measurement in the United States