Homebrew ABV Calculator

Specific gravity is the density of a liquid relative to water, and in brewing it stands in for dissolved sugar. Wort before fermentation typically reads between 1.030 for a light session beer and 1.120 for a barleywine, while finished beer settles between 1.005 and 1.020. Yeast converts glucose, maltose and maltotriose into ethanol and carbon dioxide, and because ethanol has a density near 0.789 g/mL at 20 degrees C, the loss of sugar and the gain of alcohol both push the reading downward. The widely used shortcut multiplies the gravity drop by 131.25, a factor derived empirically from ethanol density and typical wort composition. It holds well below about 6 percent ABV and drifts increasingly low above 8 percent, which is why the nonlinear alternate expression dividing by (1.775 - OG) is preferred for strong beers. Attenuation describes how far fermentation went. Apparent attenuation, computed from raw readings, usually falls between 65 and 85 percent depending on yeast strain and mash temperature; real attenuation is lower because ethanol depresses the finishing reading below what the residual extract alone would give. Practical caveats matter. A glass hydrometer resolves about 0.001, roughly 0.13 percent ABV, and it is calibrated at a fixed temperature, commonly 60 degrees F, so warm samples read artificially low. Refractometers need an alcohol correction once fermentation starts. Unfermentable dextrins from crystal malt or a high mash temperature keep final gravity elevated even when the yeast is perfectly healthy, so a high reading is not by itself evidence of a stalled fermentation.

Brewers judged strength by taste and experience until the hydrometer moved from the laboratory into the brewhouse in eighteenth-century England. John Richardson, a Hull brewer, published Statical Estimates of the Materials of Brewing in 1784 and argued for the saccharometer as a way to compare the extract yield of different malts. London porter houses adopted it quickly, since it exposed which maltsters were selling weak grain and let a brewer set a target gravity rather than guess at one. British excise law reinforced the habit by taxing beer according to original gravity, which turned accurate density measurement into a commercial necessity rather than a curiosity. Converting density into a sugar percentage came from continental Europe. Karl Balling, working in Prague, published tables in 1843 expressing sucrose content by weight; Adolf Brix refined them in the 1850s for the sugar trade, and Fritz Plato and colleagues at the German standards commission corrected them again around 1900, producing the Plato scale that most professional breweries still use. Homebrewers inherited a simplified version of all this. After the United States legalized home brewing in 1979, hobbyist manuals popularized the single 131.25 multiplier precisely because it required no tables. The Beer Judge Certification Program, founded in 1985, later fixed gravity ranges into published style guidelines, while oscillating U-tube density meters, commercialized from the late 1960s, brought laboratory-grade precision to commercial quality control.