In brewing, specific gravity is defined as the ratio of the density of a brewing liquid, such as wort or beer, as compared to the density of pure water. Typically either a hydrometer or a refractometer is used to determine the specific gravity of beer or wort.
A wide selection of home brewing hydrometers and refractometers can be found here:
The final gravity or FG of a beer is the beer’s specific gravity once fermentation has completed. Once a brewer has measured the final gravity, it can be compared to the original gravity and the ABV or alcohol by volume can be calculated. The final gravity of a beer is typically taken using a hydrometer or refractometer. Specialty final gravity hydrometers are available to allow you to take a more accurate reading of your final gravity if desired.
Below are three different types of hydrometers used for taking the specific gravity readings of beer.
Beer brewing hydrometers for taking specific gravity readings such as original gravity and final gravity.