Proof Gallon
A proof gallon is one liquid (wine) gallon of spirits at 100 proof (50% alcohol by volume) at 60°F. It is the unit the TTB uses to measure and tax distilled spirits.
Because spirits are stored and removed at many different proofs, the TTB standardizes everything to proof gallons so quantities and taxes are comparable. To convert, multiply the wine gallons by the proof and divide by 100: 10 wine gallons at 125 proof equals 12.5 proof gallons. Getting this conversion consistent everywhere is what keeps storage, production, and excise reports in agreement.
How do you calculate proof gallons?
Multiply wine gallons by the proof and divide by 100. A 53-gallon barrel filled at 125 proof holds 53 times 125 divided by 100, which is 66.25 proof gallons. The same conversion works at any proof, which is why storage, production, and excise figures can all be compared on one scale. The TTB defines the proof gallon in 27 CFR 30.36 (eCFR Title 27).
Why does the TTB use proof gallons instead of volume?
Spirits are stored and removed at many different proofs, so raw volume is not comparable between a barrel at 125 proof and a bottle at 80 proof. Standardizing to proof gallons measures the pure alcohol involved, so tax and reporting treat equal amounts of alcohol equally regardless of how diluted the liquid is.
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