Alcohol by volume (ABV) simply represents what portion of the total volume of liquid is alcohol. Our liquid of choice is, of course, beer. And to determine the ABV of a beer, a brewer typically uses what’s called a hydrometer, which is an instrument that aids in measuring the density of liquid in relation to water (it essentially free-floats in a cylinder or liquid). The hydrometer will be calibrated to read 1.000 in water (at 60°F), and the denser the liquid (example: add sugar to the liquid), the higher the hydrometer reading.
Okay, so how does this relate to beer? Well, before yeast cells are introduced to ferment beer, the liquid is called “wort (pronounced wert),” and it’s full of all kinds of sugars that were previously extracted from the grain. A brewer will take a hydrometer measurement of the wort (at 60°F) to determine what’s called the original gravity (OG). Then yeast is pitched into the wort, and fermentation begins. As the yeast cells eat the sugar in the wort, they create two wonderful by-products: carbonation (CO2) and alcohol. And once the brewer has determined that our hungry yeast have had enough (could be days, weeks or months), s/he’ll go ahead and pull another hydrometer reading (at 60°F) and record what’s called the final gravity (FG).
Notice that all measurements were taken at 60°F. That’s because the temperature of the liquid will impact the hydrometers’ measurement of the liquid, and the hydrometer was calibrated with water at 60°F. So in order to maintain controlled calculations … you get it. Otherwise you’d need to make adjustments in calculations, and we don’t want to worry about that.
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.
Often brewpubs will list the OG (sometimes called SG: specific gravity) and the FG of their beers but no estimated alcohol by volume content. Using the extremely simple formulas above, not only can you work out the alcohol by volumes of the beers, but you’ll impress the hell out of your friends.