Calculating the ABV
Say our brewer crafted a high-alcohol beer. The OG measured at 1.080, and the beer stopped fermentation with a FG measurement of 1.011. Simply subtract the FG from the OG and multiply by 131.
1.080 - 1.011 = 0.069 x 131 = 9.039%
So we've got a 9 percent alcohol by volume beer. Easy!
Alcohol By Weight
Although alcohol by volume is becoming more of a standard in the U.S., don't be fooled. Often brewers throughout the U.S. and a few parts of the world will still use what's called alcohol by weight (ABW). If you purchase a beer that has ABW listed instead of ABV, the alcohol content is going to actually be higher than you might think. To convert ABW to ABV, simply multiply the ABW by 1.25. So a 7 percent ABW beer would be a 9 percent ABV beer.
If for some reason you want to convert from ABV to ABW, multiply the ABV percent by 0.80.