During fermenation yeasts convert sugar from the wort to alcohol (ethanol) and carbon dioxide. The drop in gravity from the original to the final indicates how much of the sugar was converted. From that we can estimate the alcohol by volume since we know the difference in specific gravity of water and alcohol. Note that these reading should be taken at 60 °F (15 °C).
Specify Gravity to Alcohol by Volume Calculator
Grains and other fermenatable sugars are the back bone of a beer recipe. They provide the sugar for the yeast to convert in to alchol. They also create color and flavor.