There are several ways I have heard of to calculate the average alcohol by volume of your beer. They each give a slightly different reading but a couple of points of a percent difference really doesn’t matter.
When working out the average alcohol don’t forget to add 0.4% to your final figure to allow for the alcohol produced by your priming sugar.

METHOD ONE:

Take a hydrometer reading of the Original Gravity (O.G.) of your brew when you start brewing. Take a Final Gravity (F.G.) reading when your beer if finished and ready to bottle. Subtract the F.G from the O.G. and (ignoring the 1.) multiply by 0.135. For example 1.050 – 1.012 = 38 x 0.135 =5.13 + 0.4 = 5.53%.

METHOD TWO:

Take a hydrometer reading of the Original Gravity (O.G.) of your brew when you start brewing. Take a Final Gravity (F.G.) reading when your beer if finished and ready to bottle. Subtract the F.G from the O.G. and (ignoring the 1.) divide by 7.36. For example 1.050 – 1.012 = 38 divide 7.36 = 5.16 + 0.4 =5.46%

METHOD THREE:

Download a copy of promash from their web site (there is a link in our link section), go to the % alcohol section, input the O.G. and F.G. into the calculator and let it do the math. Remember to add the 0.4% for the priming sugar.