An extract on #demod
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.
From the 18th century until 1 January 1980, the UK measured alcohol content by proof spirit, defined as spirit with a gravity of 1213 that of water, or 923 kg/m3, and equivalent to 57.15% ABV.
The value 57.15% is very close to the fraction 47 = 0.5714. This leads to the definition amounts to declaring that 100 proof spirit has an ABV of 47. From this it follows that, to convert the ABV (expressed as a percentage standard rather than as a fraction) to degrees proof, it is only necessary to multiply the ABV by 74 . Thus pure, 100% alcohol will have 100(74) = 175 proof, and a spirit containing 40% ABV will have 40(74) = 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 was defined as 100 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).
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.
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 (OIML). 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 OIML 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.