How do you calculate the alcohol percentage in home brew beer when you do not have an initial specific gravity reading?

Ok, so you forgot to take an original gravity reading (O.G.).

Hopefully you brewed with a kit or a recipe that tells you the anticipated O.G. Get that number and then when your homebrew finishes get the final gravity reading (F.G.).

Calculate the alcohol percentage with this calculation:

% ABV = ((anticipated O.G.) - (F.G.)) / (specific gravity of ethanol)

% ABV = ((anticipated O.G.) - (F.G.)) / 0.789