Specific Gravity or Gravity, is a measure of the amount of sugar in your wort. Water has a Gravity of 1.000 so when you add sugar via malt extract or grain it rises up to greater than 1.000. Original Gravity, often referred to as OG, is the Specific Gravity you measure once your wort has chilled and before you pitch your yeast. Final Gravity (FG) is your Gravity after the yeast are done fermenting your beer. Beers typically have a final gravity between 1.005 and 1.015.
These Gravity readings are important for a few key reasons:
- They let you know if your beer is done fermenting. Once the Gravity has stabilized for a few days you can move on to bottling or kegging.
- It allows you to calculate the alcohol by volume (ABV) of your beer. (OG - FG) x 131 = ABV
- The difference between your OG and your FG (expressed as a percentage) will give you an indication of the body and dryness of your beer.