I was researching a venerable Irish whiskey recently and came across an old newspaper ad that described the whiskey as "25 degrees under proof".
These days we define alcoholic strength in terms of percentage alcohol by volume (ABV). So I wondered how to convert the older measure to its modern equivalent.
The term proof goes back a few hundred years, originating, so the story goes, with sailors suspicious that their grog ration had been watered down.
To "prove" the strength of the spirit, a dish of gunpowder was doused in the alcohol and set alight. If the gunpowder took fire, the spirit was over proof. If it did not the spirit was under proof.
Since this is not terribly scientific, parliament subsequently laid down that thirteen volumes of proof spirit is equal to the weight of twelve volumes of water, all measurements being made at 51°F.
The strength of all mixtures of alcohol and water could then be defined relative to this fixed point. They would be described as some number under or over proof, that number describing how many volumes of water should be added to or subtracted from a hundred volumes of the mixture to produce proof spirit.
Ten over proof, for example, means that ten volumes of water should be added to a hundred volumes of the spirit to make it proof.
Of course proof is defined differently in the United States. There, proof number is twice the percentage of the alcohol content measured by volume at a temperature of 60°F.
To go from US proof number to %ABV is very easy: just divide by two.
According to Merriam-Webster's Guide to Everyday Math, we can convert American proof to British proof by multiplying by 7/8.
All of which is a roundabout way of arriving at a simple formula to convert 25 degrees under proof to %ABV. We can express 25 degrees under proof as 75 proof. Convert to American proof by multiplying by 8, then dividing by 7. Then divide by 2 to get %ABV.
Or just multiply by 4/7 to get there directly. 25 under proof is the equivalent of just under 43% ABV.