Given an initial gravity of un-fermented wort (OG) and a final gravity of fermented wort (FG), better known as beer, it is possible to get the general ABV (will be a %).
Note: Any wort that gets denser through fermentation will appear to have a negative ABV; in those cases the ABV should be marked as 0. Most brews do not have a lot of room on the label so round to the 2nd decimal place. Assume a constant of 131 is involved (this is why it is only a general ABV).
x = [OG FG] = [1.085 1.009]
ABV = (OG-FG)*131 = 9.96%