ABV Calculator

Gravity-based ABV works because fermentation swaps a dense solute for a light one. Sugar dissolved in water raises specific gravity above 1.000, while ethanol has a density of about 0.789 g/mL at 20 degrees C, so as yeast converts sugar to ethanol and carbon dioxide the hydrometer reading falls. The difference between original gravity and final gravity is thus a proxy for how much sugar was consumed, and multiplying it by 131.25 is an empirical fit that translates that drop into percent alcohol by volume. Typical beer runs an OG of 1.035 to 1.080 and an FG of 1.008 to 1.018, giving 3.5 to 8 percent ABV; wine musts start near 1.080 to 1.100 and finish below 1.000 because so little residual sugar remains. Attenuation describes the same data differently. Apparent attenuation, the gravity drop divided by the original gravity excess, normally lands between 65 and 85 percent, and a reading under about 60 percent usually signals a stuck fermentation. It reads high because ethanol depresses the hydrometer, so real attenuation computed from real extract is typically several points lower. The main limitations are worth respecting: the 131.25 constant loses accuracy above roughly 1.080 OG or 8 percent ABV, which is why an alternate formula is shown alongside it; hydrometers are calibrated to a stated temperature, usually 20 degrees C or 60 degrees F, and must be temperature-corrected; and refractometers read wrongly once alcohol is present and need a separate correction. Standard-drink figures follow the US definition of 0.6 fluid ounces, about 14 g, of pure ethanol.

Measuring fermentable strength by density predates any understanding of yeast. Hydrometers were in use by brewers and excise officers well before the nineteenth century, since taxation of beer and spirits required an objective measure of strength, and the instrument gave one. The modern scales were put on a rigorous footing by Karl Balling, a chemist working in Prague, who published saccharometer tables in 1843 relating specific gravity to the percentage of dissolved sugar by weight. Around the turn of the twentieth century Fritz Plato and colleagues in Germany refined and corrected those tables, producing the degrees Plato scale that European brewers still use, where one degree Plato is one percent extract by mass. Specific gravity persisted alongside Plato in British and American practice, and the conversion polynomials between the two scales are simply curve fits to the same underlying density data. The familiar homebrew shortcut of multiplying the gravity drop by a constant near 131 emerged from mid-twentieth century brewing literature as a convenient linearization of ethanol-water density tables, accurate enough for ordinary beer strengths and increasingly wrong beyond them. Attenuation as a diagnostic became standard once brewers could compare a measured finish against a strain's published expectation. On the consumption side, the United States National Institute on Alcohol Abuse and Alcoholism formalized the standard drink at 14 g of pure alcohol, which is the basis for the standard-drink output here.