I'm trying to make wine out of pomegranate juice, its almost a month that the must is fermenting but when I read the specific gravity with the hydrometer is always around 0.008. If a try to calculate the ABV with an online calculator or with the formulas is always a negative number, am I doing something wrong??
Specific gravity is relative to water (S.G. = 1.0) so unfermented wines should be heavier/denser than water, eg S.G. 1.090, YMMV. As the wine ferments the S.G. decreases because the sugars are consumed. This makes the S.G. fall. When wine is finished fermenting it can have a S.G. close to or even below 1.000.
To calculate the (approximate) ABV one must know the original gravity and the final gravity of the fermented wine. One must record the starting/original gravity (O.G.) before/at pitching the yeast and several successive readings of the specific gravity at later dates. If the specific gravity is the same for three days in a row then the primary fermenting is deemed to have stopped. The last specific gravity reading is then the final gravity (F.G.)
One can estimate the ABV of the final brew using a formula found in this question and the original/final gravity readings noted while brewing.