Robertnbirch
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found this on wikipedia explains some things.....
For a given alcoholic beverage, there are two types of gravity measurements, taken before and after fermentation:
Original gravity (OG), which is a measure of the specific gravity of an unfermented beverage. (OG is the "sugar before fermentation" measurement.)
Final gravity (FG), sometimes called Terminal Gravity, which is a measure of the specific gravity of the fermented beverage. (FG is the "sugar after fermentation" measurement.) Typically, FG is measured only once fermentation is completely finished - that is, all of the fermentable sugars have been turned into ethanol.
The difference between OG and FG is a measure of the amount of sugar consumed in the fermentation, and therefore it can be used as a rough, indirect measurement of the alcohol content of the beverage. It is a "rough" measurement of alcohol content for several reasons:
Each yeast has a certain power of attenuation, which is the ability to consume sugars down to trace amounts.
Different beverages have higher or lower proportions of non-fermentable sugars (trisaccharides and longer polysaccharides).
Thus, final gravity is not a simple function of original gravity - that is to say, FG cannot be accurately calculated from OG. However, it is nonetheless a common practice to "design" a beverage based on the theoretical final gravity. This is done by summing the GU/lb of the beverage's various starting materials and dividing by the volume of liquid being fermented. This gives the expected final gravity, but is rarely, if ever, an accurate calculation of actual final gravity. (For example, dry malt extract has approximately 45 GU/lb. If five pounds are used to make two gallons, you have: 45GU/lb x 5lb = 225GU ; 225GU / 2gal = 112.5 GU per gallon, yielding a product with a gravity of 1.1125.)
For a given alcoholic beverage, there are two types of gravity measurements, taken before and after fermentation:
Original gravity (OG), which is a measure of the specific gravity of an unfermented beverage. (OG is the "sugar before fermentation" measurement.)
Final gravity (FG), sometimes called Terminal Gravity, which is a measure of the specific gravity of the fermented beverage. (FG is the "sugar after fermentation" measurement.) Typically, FG is measured only once fermentation is completely finished - that is, all of the fermentable sugars have been turned into ethanol.
The difference between OG and FG is a measure of the amount of sugar consumed in the fermentation, and therefore it can be used as a rough, indirect measurement of the alcohol content of the beverage. It is a "rough" measurement of alcohol content for several reasons:
Each yeast has a certain power of attenuation, which is the ability to consume sugars down to trace amounts.
Different beverages have higher or lower proportions of non-fermentable sugars (trisaccharides and longer polysaccharides).
Thus, final gravity is not a simple function of original gravity - that is to say, FG cannot be accurately calculated from OG. However, it is nonetheless a common practice to "design" a beverage based on the theoretical final gravity. This is done by summing the GU/lb of the beverage's various starting materials and dividing by the volume of liquid being fermented. This gives the expected final gravity, but is rarely, if ever, an accurate calculation of actual final gravity. (For example, dry malt extract has approximately 45 GU/lb. If five pounds are used to make two gallons, you have: 45GU/lb x 5lb = 225GU ; 225GU / 2gal = 112.5 GU per gallon, yielding a product with a gravity of 1.1125.)
