Final Gravity is a measure of the specific gravity (ratio of the density of the beer to the density of water) or apparent extract of a beer at the end of fermentation. This reading will often be taken with a hydrometer (or saccharometer). While final gravity measures the unfermented substances left in the beer, it does not take into account the fact that the alcohol produced during the fermentation is lighter than water. This skews the reading, rendering it an “apparent extract” as opposed to “real extract.” The final gravity is, however, a useful rough measure at the end of fermentation and can be used, in combination with the original gravity, to calculate the approximate ABV of the finished beer.
