Beer - how many grams of alcohol?
 

I'm not 100% on how to calculate the total grams of alcohol in a beer.

I would make an assumption that if a beer is 3.9% alcohol by volume, then take the total weight in grams of the beer, and multiply by 3.9% to determine the grams. Is this correct?

The low calorie/carb beers don't list that on their nutrition label.

Alternately, and this is what I do, you can multiply 3.9% by the total volume of the beer (355mL for a standard bottle I think), to get the volume of pure alcohol. Then multiply this by the density of ethanol, 0.789 g/mL, to get grams of alcohol.

To make things easier, you can shorten this if you only drink beer from the common 355mL bottles: just multiply the alcohol percentage by 280 (roughly .789 * 355) to get your alcohol grams. This works best for me since the alcohol in the beer I drink ranges from 4 - 10%