Original Gravity and Final Gravity

• Two measurements of density are taken in brewing: one before fermentation and one after. The measurement taken before fermentation is called original gravity (OG), and the measurement taken after is final gravity (FG). The difference between these two measurements indicates the amount of sugar used during the fermentation process.

Sugar and Gravity Measurements

• Because sugar is converted to ethanol during the fermentation process, the final gravity of a beer will be less dense than the original gravity. According to microbrewery Brownie's Ales, sweeter beers, in which not all of the sugar has been converted, have a lower original gravity.

The Gravity of Various Types of Beer

• Milds beers and wheat beers have 1.020-1.040 OG. Lagers, stout, porter, pale ale, and bitters generally have an OG of 1.040-1.050. ESB, IPA, and beers marketed as Oktoberfest beers normally come in at 1.050-1.060 OG, while strong ales and bocks have 1.060-1.075 OG. The beers with the lowest OG include barley brews and Belgian trippels, often with less than 1.075 OG.

Alcohol by Weight

• To use these measurements to find the percentage of alcohol in beer, subtract the TG from the OG and multiply by 1.05 (the amount of carbon dioxide made for every gram of ethanol produced during fermentation). For example, if your TG is 1.35 and your OG is 1.75, subtract 1.35 from 1.75 to get .4. Multiply .4 by 1.05 to get 4.2 percent alcohol by weight.

Alcohol by Volume

• To find the amount of alcohol by volume (as shown on beer cans and bottles), take the by weight percentage and divide by the density of alcohol, set at 0.79 kg per liter. To continue the example above, divide 4.2 by 0.79 to get 5.3 percent alcohol by volume.