Alcohol by Volume from Gravity
The alcohol by volume of a fermented beer, wine, cider, or mead from gravity readings -- the brewer's end-of-fermentation number (distinct from the cocktail-dilution ABV tile). Specific gravity is density vs water (1.000); sugars push the ORIGINAL gravity above 1.000, and as yeast makes alcohol the gravity falls to the FINAL gravity. ABV% = (OG - FG) x 131.25: a beer 1.055 -> 1.012 drops 0.043 = ~5.6% ABV. Apparent attenuation = (OG - FG)/(OG - 1) = 0.043/0.055 = 78% (how much extract fermented). A 1.065 -> 1.010 wort is ~7.2% ABV, 85% attenuated. The 131.25 factor is an approximation that drifts high on strong brews (a more elaborate formula tracks better). A working estimate; a temperature-corrected hydrometer/refractometer and, for a sold product, the TTB/lab method govern the label.
Formula and source
abv_pct = (original_gravity - final_gravity) x 131.25; apparent_attenuation_pct = 100 x (original_gravity - final_gravity) / (original_gravity - 1).
Alcohol by volume from gravity (standard homebrew/brewing formula; Papazian, The Complete Joy of Homebrewing; ASBC context), by name; a temperature-corrected hydrometer or refractometer and, for a sold product, the TTB / laboratory method govern the label value.
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