THE RELATION BETWEEN PROOF AND ABV PERCENTILE DEMYSTIFIED
Alcohol strength is measured across the world according to terms framed by the International Organization of Legal Metrology (OIML). Alcohol content of a beverage is measured by distilling off the alcohol, measuring the volume of alcohol distilled and expressing it as a percentage of alcohol by volume (ABV) at a temperature of 20° C (68° F). Weighing the alcohol distilled vis-à-vis original weight would provide percentage of alcohol by weight (ABW). ABW can be converted to ABV by dividing ABW by 0.79 (taken as 0.80). So, ABW=0.8 ABV. Thus 3.2 ABW = 4.0 ABV.
The whisky industry has agreed to a global constant in terms of alcohol by volume, ABV. ABV is also called Gay Lussac (GL), an abbreviation of the French chemist and physicist Gay-Lussac’s name. Joseph Louis Gay-Lussac (1778 - 1850) is known mostly to the alcohol industry for his work on alcohol-water mixtures. Gay-Lussac devised a scale for measuring the quantity of alcohol by volume for the French wine industry, extending to other French liquor like Brandy, Cognac and Armagnac. Its accuracy led to the scale being used to measure alcoholic beverages in many countries.
The OIML scale and the Gay-Lussac scale are basically the same; both measure alcohol by volume but the Gay-Lussac results are expressed in degrees, not percentages. The alcohol is quantified by a hydrometer and, based on volume, each degree Gay-Lussac is rated as one percentage point. A drink with an alcohol content of 40 percent by volume would rate 40 degrees Gay-Lussac (40°GL), or 40% ABV. The only difference between ABV and GL is that GL uses the International Standard definition of atmosphere, where ambient temperature is taken as 15°C. ABV uses 20°C as its base value, in keeping with industry norms.
The UK measured alcohol content in terms of ‘proof spirit’ up to 01 January 1980, while also adding the ABV figure. The Scotch Whisky Association, using a scale first delineated by one Bartholomew Sikes in 1816, defined ‘proof’ as spirit with a gravity of 12/13 that of water, or 923 kg/m3 and 100° proof was equivalent to 57.15% ABV, which definition was accepted throughout the UK, as verified later by Gay-Lussac, but not in the US.
Degrees proof has a naval link, though the Army also claims its share. In the 18th century, payments to British sailors/soldiers included daily tots of rum, or ‘grog’. The rum would be ‘proved’ acceptable by pouring it on gunpowder and lighting it. If it burned steadily with a blue flame, it was considered 100 degrees ‘proof’ and equalled 57.15 alcohol (chemical name: ethanol). If it failed to burn, it was considered ‘under proof’ and evidence that it had been adulterated. If it burned too quickly, it was ‘over proof’, not that the sailors minded this lip-smacking occurrence. Rum that contained the desired percentage of alcohol was defined to be ‘100° (one hundred degrees) proof’.
The Yanks differ. They say that it was their soldiers in the Civil War who did the gunpowder trick. Brit soldiers in the US War of Independence also lay claim to this process. That is no longer relevant.
While the term "proof" stuck, in America, the standard it refers to has nothing to do with gunpowder. Around 1848, 50% alcohol by volume was chosen as a baseline and 100 was used as its corresponding proof. Thus, the proof is double the ABV.The unit ‘degree’ has been done away with.
The figure 57.15 per cent approximates the fraction 4/7 (0.5714). Therefore, to convert the ABV (expressed as a percentage rather than as a fraction) to degrees proof, it is only necessary to multiply the ABV by 7/4 (1.75). 100 percent ABV alcohol will thus be 100 × (7/4) = 175° proof, and a spirit containing 50% ABV will be 50 × (7/4) = 87.5° proof. Some bottlers used to label their product 25° under proof, for reasons best known to themselves. Inexplicably, this means 100-25=75° proof, not 175-25=150° proof and definitely not 200-25=175° proof. This is converted into ABV by multiplying it by 4/7. 75° proof is thus 75 x 4/7 = 42.8 per cent ABV.
Proof value in the US is exactly twice the ABV, which makes sense. US Regulations do not bar mention of the proof value, as long as it is printed close to the ABV, successfully complicating the issue if you are European. 200 proof alcohol (pure ethanol) can be found only inside a laboratory, because pure alcohol, when exposed to free air, will absorb moisture from the atmosphere and self-dilute down to about 194 proof. The American standard bottle size is 750 ml; standard size in the EU is 700 ml. Bottlers of Scotch whisky destined for the American market have to buy their bottles from US manufacturers.