Alcohol proof
Alcohol proof is a measure of the content of ethanol (alcohol) in an alcoholic beverage. The term was originally used in the United Kingdom and was equal to about 1.75 times the alcohol by volume (ABV). The UK now uses the ABV standard instead of alcohol proof. In the United States, alcohol proof is defined as twice the percentage of ABV.
The measurement of alcohol content and the statement of content on bottles of alcoholic beverages is regulated by law in many countries.
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. 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. 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 12⁄13 that of water, or 923 kg/m3, and equivalent to 57.15% ABV.
The value 57.15% is very close to the fraction 4⁄7 = 0.5714. Thus, the definition amounts to declaring that 100° proof spirit has an ABV of 4⁄7. 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 by 7⁄4 = 1.75. 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 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).
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