Expert: Spencer W Thomas - 4/24/2007

Question
My question is how to you figure ABV when you add fermentables to your secondary such as maple syrup, honey ,etc? Do you take a SG reading before your tranfer to the secondary and a new one afterward?  Example: OG is 1.040 and before tranferring it to the secondary it is 1.010 and then when you put in your new fermentable the SG has gone up to 1.020 and then at bottling the SG is 1.015 Is the ABV figured as a combination of the two SG drops....030 + .005 = .035 for a ABV of .035 * 129 = 4.5% ABV.....these are just made up readings,,,,
Thanks,
Brian  

Answer
That would work reasonably well.

If you're using something like fruit pulp, though, it won't because at least some of the sugar in the fruit pulp doesn't contribute to the SG reading, and because the pulp itself may be in suspension and therefore affecting the reading.

There are methods to measure ABV that rely on boiling off the alcohol and measuring the weight difference of the liquid before and after (making the final liquid back up to the original volume) that could be used in the latter case, but they're fiddly (volume has to be measured quite precisely, for example.)