Since % ABV stands for the Percentage of Alcohol by Volume - it only stands to reason that adding syrup or juice concentrate after fermenting will change that ratio. If you add more liquid (thereby increasing the volume) but you don't add any additional alcohol - you have decreased the % of alcohol in the new, larger volume. I got some help with the math on the Home Winemaking FB page and put together a spreadsheet to assist others with the calculations. As I understand it - to calculate the ABV of your wine when it is dry - you take the beginning SG, subtract the ending SG (dropping the decimals) then divide the result by 7.36. So - starting SG @ 1.090 > ending sg .990 = 1090 - 990 = 100. Divide 100 by 7.36 = 13.58% ABV. I wanted to know because I have 5.5 gal of Dragon's Blood and my notes say last time I added 6 cans (12 oz each) of juice concentrate to backsweeten and boost the flavor. That's 72 oz (6 X 12) added to 704 oz (5.5 gal X 128 oz per gal) - almost a 10% increase in volume. So now I have 776 oz (a bit over 6 gal) - but the same amount of alcohol. What would the NEW % ABV be?? If you input your starting and ending SG in the highlighted boxes (w/o decimal points) and your volume BEFORE adding sweetener and the volume of sweetener - it should do the calculations for you. Unfortunately - I don't seem to be able to upload either and Excel spreadsheet or a Libre open source document - (.ods) So - here's a link to get to it. It's in LibreOffice and Excel format for those of you that are Microsoft hostages. Let me know if you can't open it. http://www.simpsontaxidermy.com/WinePage.html Hope it helps someone.