Hi All, I thought I'd post how I calculate how much brandy to add to port to boost the alchohol: P=Port at start (what you made) a=alchohol percentage of port you made (SG-FG)*131 b=alchohol percentage of brandy you are adding A=final percentage alchohol you wish to have B=brandy to add (what we are solving for) P*a+B*b=(P+B)*A P*a+B*b=P*A+B*A B*b-B*A=P*A-P*a B*(b-A)=P*(A-a) B=P*(A-a)/(b-A) Example: P=12L, a=16%, b=40%, A=20% B=12*(0.2-0.16)/(0.4-0.2) = 2.4L brandy to add.

Nice. Or you can use a Pearson Square: https://winemakermag.com/article/the-pearson-square There are also times that one would like to make a port as they do in Porto (or Oporto), that is, by adding the fortifying alcohol during an active fermentation, so that the increased ABV stops the fermentation and leaves the desired amount of residual sugar (and the desired ABV). I worked out the math for that (quoted below) here: https://www.winemakingtalk.com/threads/whole-grape-port.44373/page-3#post-503223 I also wrote up a spreadsheet that will make this calculation for you: Here is the calculation in all of its, ummm, glory:

The Pearson Square seems to go off the same type of formula's as mine (i checked and the math works out the same with both), but what I don't like is you have to know what your final quantity will be, whereas my calculation tells you the quantity to add just to boost the ABV of the batch. For some reason it confuses me as I'm better with algebra than drawing lines to perform calculations lol.