Originally Posted by Daze
look at the grams of sugar on the juice. About 450g per gallon will give you 5% so if the container is 1 gallon and the serving size is 8 oz take the g of sugar per serving and multiply it times 16 to get the total grams in the juice. a lot of grape juice is about 40g of sugar per 8oz serving so that would make a gallon 640g resulting in an ABV of approximately 7% that means you will need to add another 450g of sugar to get about 12% From there you can take an SG reading and add sugar as needed to get the extra .5%
I'm not sure how you calculated this, but my calcs would be over 14% for that much sugar. For example, I believe 640 grams of sugar in a gallon has potential for approx 8.5%.
How much sugar to add to 5 gallons of juice at 1.060 to get 12.5% ABV:
12.5% ABV requires an OG of approx 1.095
. Your juice is at 1.060
, so you need to raise the gravity by 35 points (95-60=35). Batch size is 5 gallons, so you need 35 * 5 = 175 points total. One lb sugar is 45 points. 175 / 45 = 3.88 or approx 4 lbs sugar to raise the gravity of 5 gallons 35 points. Make sense? This assumes you ferment to dry. Stopping fermentation early to retain sweetness would require more sugar.