Alcohol By Volume (ABV) Calculator

Specific gravity (SG) is the density of a liquid compared to pure water at the same temperature. Water has an SG of exactly 1.000. Sugar dissolved in wort raises the density, so a typical pre-fermentation original gravity (OG) reads 1.040 to 1.080. As yeast convert sugar to alcohol and CO2, density drops, and a final gravity (FG) of 1.008 to 1.020 is normal for most beer styles. The difference between OG and FG is the input every ABV formula needs.

A hydrometer is a sealed glass float with a weighted bulb and a calibrated stem. You drop it into a test sample of your wort and read where the liquid surface crosses the stem. The denser the liquid, the higher the hydrometer floats, which corresponds to a higher gravity reading. Most brewing hydrometers are calibrated to 60 F (15.5 C) or 68 F (20 C), so adjust the reading if your sample is significantly warmer or colder. A refractometer reaches the same answer with a single drop, but its readings get distorted by alcohol, so most brewers still verify FG with a hydrometer.

The simple formula, ABV = (OG - FG) * 131.25, is fine for everyday session beers and pale ales below about 6% ABV. Above that the simple formula starts to under-report because it ignores how ethanol changes the density of the fermenting liquid. The advanced formula corrects for that and is reliable up to roughly 8% ABV, which covers most IPAs and Belgian ales. For 8 to 12%+ beers, like imperial stouts, barley wines, and Belgian quads, the Miller alternative formula tracks the real fermented alcohol more accurately. Picking the wrong formula on a big beer can mislabel the bottle by a full percentage point.

Apparent attenuation, (OG - FG) / (OG - 1) * 100, is the percentage of sugars the yeast appears to have fermented, calculated straight from gravity readings. It runs high because ethanol is less dense than water, so it pulls FG down further than the sugar reduction alone would. Real attenuation is roughly apparent attenuation * 0.81 and represents the actual percent of sugars the yeast consumed. A typical ale finishes at 75% apparent and about 60% real attenuation. Pro brewers and yeast labs publish both, but apparent is what you see on a homebrew hydrometer.

US proof is exactly twice the ABV by definition. A 5% ABV pale ale is 10 proof, a 12% imperial stout is 24 proof, and a 40% bourbon is 80 proof. The UK historically used a different proof system based on gunpowder ignition (100 UK proof equals about 57.1% ABV), but that scale was abandoned in 1980 and the US 2x ABV definition is the only one still in active use. The calculator shows US proof alongside ABV so you can label a homebrew or compare beer strength to a spirit on the same scale.