What is the difference between a "wine gallon" and a "proof gallon"?

A "wine gallon" is a measurement of any liquid's total volume.  1 "wine gallon" is equal to 128 fluid ounces or 3.78 liters.  

A "proof gallon" is a measurement of only the ethyl alcohol contained in a given volume of liquid.  1 proof gallon is equal to 1 wine gallon of liquid but containing only 50% ethanol.  The other 50% is typically water, but can technically be any liquid so long as it is not ethanol.

See below for examples to demonstrate the point.

• To Recap:
  • 1 Wine Gallon = 1 Volumetric Gallon = 128 fluid ounces of liquid.  
  • 1 Proof Gallon = 1 Volumetric Gallon containing 50% ethyl alcohol.  
• ​​If your mother asked you to bring home a gallon of milk from the store, you would go to the grocery store and pick up 1 "wine gallon" of milk to bring home. 

Because a proof gallon is dependent on the amount of ethyl alcohol present, 1 proof gallon can be represented by many different final volumes of liquid.  For example:

• 1 wine gallon at 50% Alcohol By Volume (ABV)
• 2 wine gallons at 25% ABV
• 1/2 of a wine gallon at 100% ABV

Because proof gallons are useful in measuring the amount of ethyl alcohol present in a contained volume of liquid, Federal excise taxes are calculated based on proof gallons.

Some more proof gallon examples:

• A 750ML bottle of 80 proof vodka contains 0.158 Proof Gallons of ethyl alcohol.
• A 12oz can of lite beer at 5% ABV contains 0.0009375 Proof Gallons of ethyl alcohol.
• A 55 gallon drum of 190 proof Culinary Solvent contains 104.5 Proof Gallons of ethyl alcohol.
• A 5 gallon jug of 200 proof Culinary Solvent contains 10 proof gallons of ethyl alcohol.