The volume of alcohol in a finished wine is directly related to
how much fermentable sugar (by weight) the must contains.
A triple-scale hydrometer (with Specific Gravity, Potential
Alcohol, and Brix/Balling scales) lets you read the predicted
value directly. Let’s say you have a juice that is 12 Brix and
you want to add enough sugar to it to bring it up to 24 Brix.
You can make a sugar solution of known concentration
(discussed in next month’s article) and determine how much
syrup to add. Of course, part of the sugar solution is water,
which increases the must’s volume.