Take a hydrometer reading right before you pitch the yeast into your wort (see our article "How to take a hydrometer reading" ). Make a note of this reading, which will vary by style but should be larger than 1.040.
After the fermentation is complete, take another hydrometer reading right before you bottle. Be sure you take your reading BEFORE you add the priming sugar, as this will skew the reading. This reading will usually fall between 1.010 and 1.030, again depending on the recipe and style. Then simply subtract the second reading from the first and multiply by 131. This will give you a very close approximation of your alcohol by volume.
Example:
You can determine alcohol content by subtracting your final gravity from your starting gravity, and multiply the result by 131.
(SG-FG)x131=ABV
For example your starting gravity is 1.060 and your final gravity is 1.012:
1.060-1.012=0.048
0.048 x 131= 6.3% ABV