A proof gallon , also called a “tax gallon,” is the measurable unit used for determining taxation, but the calculation varies per country. A proof gallon is one US gallon at 50 percent alcohol by volume. Thus a gallon of 151-proof rum is 1.51 proof gallons, while a gallon of, say, Bénédictine, at 40 percent ABV, is 0.8 proof gallons. For the United Kingdom and Canada, a proof gallon was formerly an imperial gallon of 4546 cm3 (277.4 cubic inches) at 50 percent alcohol by weight, or 57.1 percent alcohol by volume. At present, however, taxation in those countries is “per liter of absolute alcohol”—in other words, on just the alcohol contained in the beverage.
In 2017, the US excise tax was $13.50 per proof gallon.
See also excise, taxes, and distillation; and proof.
Tax and Trade Bureau FAQs on Proof Gallons. https://www.ttb.gov/images/formstutorials/f511040/faqinstructions.html (accessed March 8, 2021).
By: Fred Minnick