You can easily establish the approximate ABV % (alcohol by volume) of your beer by using a hydrometer. A hydrometer is a tool which measures the density or specific gravity of a liquid. To work out the ABV % of your beer, simply subtract FG (final gravity) from OG (original gravity) and multiplying by 131.25.
ABV = (OG-FG) x 131.25
ABV = (1.040-1.010) x 131.25
ABV = 0.030 x 131.25 = 3.937%
OG stands for Original Gravity – This refers to the ‘original’ or starting gravity of the wort before fermentation commences.
FG stands for Final Gravity – This refers to the final or finishing gravity once fermentation is complete. It is also sometimes referred to as the terminal gravity. This value also allows you to determine whether fermentation has completed.