Wine is (some not all) is 12% alcohol by vol.

Just to see how different % volume and % alcohol are let's determine the number of grams of alcohol in a 750 mL bottle.

First let's assume the wine is12% alcohol by weight and second let's assume the wine is12% alcohol by volume. (We will need to know the density of ethanol=0.789 g/ml, density of wine which we will estimate is 1.00 g/ml ).

12% by volume&emdash;compares vol wine to vol alcohol.

Convert volume of wine to volume of alcohol. Convert volume of alcohol to grams of alcohol.
 

12% by weight&emdash;compares wt wine to wt alcohol.

Convert volume of wine to weight of wine. Convert weight of wine to weight of alcohol. 

Clearly, 12% by weight would be different than 12% by volume

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