<P> From the 18th century until 1 January 1980, the UK measured alcohol content by proof spirit, defined as spirit with a gravity of ​ ⁄ that of water, or 923 kg / m, and equivalent to 57.15% ABV . </P> <P> The value 57.15% is very close to the fraction ​ ⁄ = 0.5714 . This leads to the definition amounts to declaring that 100 ° proof spirit has an ABV of ​ ⁄ . 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 ​ ⁄ . Thus pure, 100% alcohol will have 100 × (​ ⁄) = 175 ° proof, and a spirit containing 40% ABV will have 40 × (​ ⁄) = 70 ° proof . </P> <P> 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 . </P> <P> 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). </P>

100 proof rum contains what percentage of alcohol