ABV / Attenuation Calculator

How this works

Alcohol by volume is derived from the drop in specific gravity between your original gravity (OG, measured before pitching yeast) and final gravity (FG, measured once fermentation is complete) — the more sugar the yeast converted, the further gravity fell, and the more alcohol is present.

This calculator computes ABV two ways, deliberately shown side by side rather than picking one.

The simple formula — ABV = (OG - FG) x 131.25 — is the formula most homebrewers learn first. It's fast and accurate to within roughly two tenths of a percent at normal beer strengths, but it's a linear approximation of a relationship that isn't actually linear, and it drifts increasingly high above about 1.075 OG.

The advanced formula — ABV = (76.08 x (OG - FG) / (1.775 - OG)) x (FG / 0.794) — is commonly attributed to Michael L. Hall's 1995 article in Zymurgy and is reproduced in most modern brewing software. It tracks more accurately at higher gravities because it accounts for how the density of an alcohol/water/extract mixture actually changes as fermentation progresses, rather than treating the gravity drop as a straight line.

At typical session and standard-strength beer gravities, the two formulas usually agree within a couple tenths of a percent. The gap widens noticeably on strong beers — barleywines, imperial stouts, high-gravity Belgian styles — which is exactly where getting the number right matters more, not less.

Apparent vs. real attenuation

Attenuation is the percentage of a wort's fermentable extract that yeast actually converted. Apparent attenuation — (OG - FG) / (OG - 1) x 100 — is calculated directly from your two gravity readings and is the number most brewers mean day to day when they say "attenuation."

Real attenuation is always a few points lower than apparent attenuation for the same OG/FG pair, because a hydrometer reading taken on beer containing alcohol reads artificially low: ethanol is less dense than water, so a wort that's already partly converted to alcohol reads a lower gravity than its actual remaining sugar content would suggest on its own. This calculator corrects for that using the Balling real-extract formula (RE = 0.1808 x OE + 0.8192 x AE, worked in degrees Plato), which is the standard correction used across brewing science and software.

Calories

The estimated calorie count uses the same real-extract terms, via the standard homebrew calorie formula built on alcohol-by-weight and real extract. Treat it as an estimate for a homebrewed beverage, not a certified nutrition label — it doesn't account for anything added after primary fermentation (fruit, lactose, spices) unless you factor that into your gravity readings yourself.

Reading the result

If your ABV looks lower than you expected, check your attenuation first — a stuck or sluggish fermentation (underpitched yeast, too-cold fermentation temperature, or a strain simply not suited to your grain bill) is the most common cause, and the gap between apparent and real attenuation will tell you whether your hydrometer reading is being distorted by dissolved alcohol or whether fermentation genuinely stopped early.

This tool is about measurement, not encouragement to push strength as high as possible — see the responsible-brewing note below every result. Homebrewing and its resulting beverages are intended for adults of legal drinking age in their jurisdiction.

A worked example

Take a common American ale reading: OG 1.052, FG 1.012. The simple formula gives (1.052 - 1.012) x 131.25 = 5.25% ABV. The advanced formula gives a close but not identical result at this moderate gravity — 5.36%, about a tenth of a point higher — both well within the range the American IPA and American Pale Ale style guidelines describe. Apparent attenuation works out to (1.052 - 1.012) / (1.052 - 1) x 100, or roughly 77%, right in the middle of a clean American ale yeast strain's typical published range. Real attenuation comes out to 62.4% once the Balling correction is applied — a good 14-point gap from the apparent figure, which is the expected, correct behaviour at this attenuation level, not a sign anything is wrong with either your reading or the calculator.

Where a bad reading hides

If your ABV comes out implausibly high or low compared to the style you're brewing, check your gravity readings before assuming the yeast did something unusual — an uncalibrated hydrometer, a reading taken without correcting for sample temperature, or a reading taken from a sample that still had suspended CO2 in it (which artificially inflates an apparent gravity reading) are all more common causes of a surprising ABV number than genuinely unusual fermentation behaviour.