Am I messing these calculations up somewhere? If I calculated correctly, a 12 oz. can of Bud gets about 95 of its 145 calories from alcohol.
A 12 0z. serving of Budweiser at 5% acl. by volume (4% by weight), has about .48 oz of alcohol. .48 oz. equals 13.6 grams. One gram of alcohol has 7 calories. 13.6 x 7 = 95.
The only problem I can see is that 12 Oz. would be a measure of volume not weight. so that would convert to .60 fluid Oz. of alcohol that you would have to convert to weight ounces before converting to grams and continuing. That would be the .48 you have listed only if the beer weighed 12 Oz. as well as had a volume of 12 Oz.
This is another reason I like metric measures better ounces are used to measure too many different things.
I found a bit more. According to this site alcohol weighs 0.79 kg/L. So .60 fluid ounces converts to roughly .0177 L and would weigh around 14 grams and that gives you 98 calories. So long story short: flawed method but nearly the right answer anyway. In fact considering the % is probably only accurate to 1 digit both answers should likely be rounded to 100 calories and called good.