History
From the 18th century until 1 January 1980, the United Kingdom measured alcohol content in terms of "proof spirit", which was defined as spirit with a gravity of 12/13 that of water, or 923kg/m3, and equivalent to 57.15% ABV. The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was "proved" by dousing gunpowder in it, then tested to see if the gunpowder would ignite. If it did not, then the rum contained too much water and was considered to be "under proof". Gunpowder would not burn in rum that contained less than approximately 57.15% ABV. Therefore, rum that contained this percentage of alcohol was defined to have "100° (one hundred degrees) proof".
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, as is 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 50% ABV will have 50×(7/4) = 87.5° 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).
Many countries also use a measure called a standard drink. In Australia, a standard drink contains 10 g (12.67 ml) of alcohol, the amount that an average adult male can metabolise in one hour. The purpose of the standard drink measure is to help drinkers monitor and control their alcohol intake.
Read more about this topic: Alcohol Proof
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