Simple subtraction. Take a hydrometer reading right before you pitch the wine yeast into your must (see “How to Take a Hydrometer Reading” ).
Make a note of this reading, which should be in the range between 1.060 and 1.120, depending on the recipe and style you are making.
After the fermentation is complete, take another hydrometer reading right before you bottle. This reading will usually fall between 0.090 and 1.010, again depending on the style of wine made.
Then you simply subtract the second reading from the first, and multiply by 131 and this will give you a very close approximation of your alcohol by volume. For example- your starting gravity reading is 1.095 and your final gravity reading is 0.995 the math would look like this:
(1.095 - 0.995) X 131= 13.1, your alcohol by volume would be approximately 13.1%.
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