What is a proof gallon?
According to the Federal TTB: "A proof gallon is one liquid gallon of spirits that is 50% alcohol at 60 degrees F. Distilled Spirits bottled at 80 proof (40% alcohol) would be 0.8 proof gallons per gallon of liquid... One gallon of alcohol that is 200 proof is equal to 2.0 proof gallons."
Frequently Asked Questions: TTB Form 5110.40
A regular gallon is sometimes referred to as a "wine gallon".
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