This is a great question. Luckily the answer is simple. You still only calculate potential alcohol based on the original Brix reading. “Negative Brixes,” or when the density of your fermented solution reads below the 0.00 °Brix mark on your hydrometer, happen because they are just that: Fermented. Alcohol is much less dense than water and when we come to the end of the fermentation and the sugar has almost all been converted to alcohol, Brix is truly only a measurement of solution density. As the relatively more-dense water and sugar solution (grape juice) gets transformed into a water and ethanol solution (wine) the new alcohol content skews the results artificially lower. There is a significant contribution from the alcohol to the ending Brix reading, which means that by the end of fermentation the Brix “reading” itself is artificially low. This means that when calculating potential alcohol we do not count negative Brix degrees as actually contributing to the alcohol level. In a way, they are like “phantom Brix units” that are really meaningless. Estimating potential alcohol is tough, even
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