Alcohol proof

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, became common. In England spirits were originally tested with a basic "burn-or-no-burn" test, in which alcohol that would ignite was said to be "above proof" and alcohol which would not was said to be "under proof".[1] The breakpoint between under and over proof was defined as 100 and was the basis for taxation. Because alcohol's flammability is highly dependent on temperature, 100 proof defined this way could range anywhere from 40 to 90% ABV in normal air temperatures; at 22 °C (72 °F) 100 proof would be 60% ABV.[2]

Another early method for testing liquor's alcohol content was the "gunpowder method." Gunpowder was soaked in a spirit, if the gunpowder could still burn the spirit was rated above proof. This test relies on the fact that potassium nitrate (a chemical in gunpowder) is significantly more soluble in water than in alcohol.[3] While less influenced by temperature than the simpler burn-or-no-burn test, gunpowder tests also lacked true reproducibility. Factors like 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 for defining proof. However, it was not until 1816 that a legal standard based on specific density was defined in England. 100 proof was defined as a spirit with ​¹²⁄₁₃ the specific gravity of pure water at the same temperature.[4] From the 19th century until 1 January 1980, the UK officially measured alcohol content by proof spirit, defined as spirit with a gravity of ​¹²⁄₁₃ that of water, or 923 kg/m³, and equivalent to 57.15% ABV.[5]

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.[4] 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).

• Liquor
• Cask strength
• Volume percent