How much sugar?

I generally make 5gallons of wine from bottled grape juice. Is there simple formula to calculate how much sugar to add during primary fermentation to get 12.5% alcohol at final bottling? My beginning begenning SG is always 1.06

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%

Daze said:

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%

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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.

450 grams per gallon or about a pound gets you about 5% I do the math every time I start a batch and I back it up with both a hydrometer and refractometer readings.

640g / 450g = 1.422

1.422 * 5% = 7.11% based on the juice alone

7.11% + another 5% by adding another pound of sugar will result in 12.111%

Like I said above I know that 450g per gallon results in about 5% because I have backed it up with test equipment.

Daze said:

450 grams per gallon or about a pound gets you about 5%

Like I said above I know that 450g per gallon results in about 5% because I have backed it up with test equipment.

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Here's where we differ. How do you know that one pound sugar per gallon gives 5%? Do you have a source or just your observations?

I believe a pound of sugar per gallon has the potential for a little over 6%. I confirmed by using http://www.gotmead.com/index.php?option=com_content&task=view&id=745&Itemid=16

up my bad you are correct and I am wrong 450 grams is closer to 6% so he would be at 8.5% to begin with and need another 300g per gallon to get the ABV he is looking for (assuming git gets fermented out dry)

some how I had 5% stuck in my mind in stead of the 6% I knew it was.