In brewing attenuation is the percentage that measures the conversion of sugars into alcohol and carbon dioxide by the fermentation process. The percentage is calculated by comparing weight or specific gravity of the extract before and after fermentation.
This formula works with extract given in weight percentages or degree Plato. Extract refers to all the non water substances (sugars, dextrins, proteins, vitamins, minerals, etc.) that are present in the wort. The percent extract or Plato scale is a measure in percent of how much of the wort’s weight is extract. Since, at least in the wort and beer gravities that most brewers work with, an almost linear relationship between (specific gravity - 1) and extract percentages exists, the above formula can also be expressed as:
for brewers who prefer to work with specific gravity.