ABV Calculator (Alcohol by Volume)

ABV Calculator

What OG and FG actually measure

Specific gravity is the density of your wort or beer relative to pure water, which reads 1.000. Dissolved sugar raises density, so pre-fermentation wort is heavier than water: most ales start with an original gravity between 1.040 and 1.070, and a 1.048 wort is roughly 12 degrees Plato (gravity points divided by 4). During fermentation, yeast converts that sugar into ethanol and CO2. Ethanol is less dense than water — 0.789 g/mL at 20°C — so gravity falls for two reasons at once: sugar leaves and alcohol arrives. Typical ale final gravity lands between 1.008 and 1.016. FG can even finish below water’s 1.000: the BJCP 2021 guidelines list American Light Lager as low as 0.998, because a fully attenuated, alcohol-thinned beer holds almost no residual sugar. The gap between those two readings is the raw material for every number the calculator above reports, including apparent attenuation — (OG − FG) ÷ (OG − 1) — which tells you how completely your yeast finished the job. Most ale strains land between 70% and 80% apparent attenuation; a beer stuck well below that range usually points to a fermentation problem, not a formula problem.

How to calculate ABV: why (OG − FG) × 131.25 works — and when it drifts

The 131.25 constant compresses real chemistry into one multiplier. When yeast ferments sugar, roughly half the consumed mass escapes as CO2 gas, and the resulting gravity drop is proportional to the ethanol created. Fold in ethanol’s density of 0.789 g/mL and fermentation’s sugar-to-alcohol yield, and you get about 131.25 percentage points of ABV per full point of gravity drop. For session-strength beers the shortcut is accurate to within roughly 0.2 percentage points. The catch is that the relationship between gravity and alcohol isn’t truly linear, and the simple formula underestimates as the numbers climb. That’s why the calculator also runs the equation Michael Hall published in Zymurgy in 1995: ABV = 76.08 × (OG − FG) ÷ (1.775 − OG) × (FG ÷ 0.794). On a 1.050-to-1.010 pale ale the two formulas differ by about 0.1 points — noise. On a 1.100-to-1.020 imperial stout, the standard formula says 10.5% while the Hall formula says 11.6% — a full percentage point of real alcohol the shortcut missed. Above roughly 6.5% ABV, log the alternate number; below that, either formula is fine.

Worked examples from real gravity readings

Take the classic homebrew example: OG 1.050, FG 1.010. The gravity drop is 0.040, so ABV = 0.040 × 131.25 = 5.25%. Apparent attenuation is 0.040 ÷ 0.050 = 80%, and the calorie estimate comes out near 165 calories per 12 oz pour — between a mass-market lager and a typical craft pale ale. Now an American IPA at OG 1.062, FG 1.012: the standard formula gives 6.56% while the high-gravity formula gives 6.80%, a gap you can already see at this strength. Push into imperial stout territory — OG 1.100, FG 1.024 — and the standard result of 9.98% understates the Hall result of 11.0% by more than a full point, with 76% apparent attenuation. Calories scale fast too: that stout estimates around 350 calories per 12 oz, more than double the pale ale. Enter your own two readings in the calculator above and compare the two ABV lines. If they diverge by more than a couple of tenths, your beer is strong enough that the alternate formula is the one worth writing in your brew log.

Measuring gravity correctly: hydrometers and refractometers

Both formulas are only as good as your two readings. Hydrometers are calibrated to read true at one temperature — 60°F (15.6°C) on most older instruments, 68°F (20°C) on many newer ones; the calibration temperature is printed on the paper scale inside the stem. Read a warm sample and the instrument lies low: wort at 80°F on a 60°F-calibrated hydrometer reads about 0.002 under its true gravity, an error worth roughly 0.26 ABV points if it hits only one of your readings. Cool samples toward calibration temperature, or run them through the hydrometer temperature correction calculator before entering numbers here. Spin the hydrometer to knock off clinging CO2 bubbles and read at the bottom of the meniscus, at eye level. Refractometers need only a few drops and work well for OG — multiply Brix by about 4 for gravity points, after dividing by a wort correction factor near 1.04. After fermentation they’re a trap: ethanol bends light more than the sugar-based scale assumes, so an uncorrected post-fermentation reading can show around 1.020 when a hydrometer shows 1.010. For FG, use a hydrometer or apply an alcohol-correction conversion first.

ABV targets by beer style

The BJCP 2021 style guidelines publish vital statistics for every recognized style, and they’re a useful sanity check on your calculated number. American Light Lager runs 2.8–4.2% ABV from an OG of just 1.028–1.040, finishing as low as 0.998. American IPA runs 5.5–7.5% from OG 1.056–1.070 and FG 1.008–1.014 — note that the upper half of that range sits above the 6.5% mark where the high-gravity formula starts to matter. Imperial Stout spans 8.0–12.0% from OG 1.075–1.115 and FG 1.018–1.030; at those gravities the standard formula can under-report by a point or more. If your result lands outside the window for the style you brewed, suspect measurement before recipe: a stuck fermentation leaving an ale at 1.020 or higher, an uncorrected warm hydrometer reading, or an uncorrected refractometer FG are all far more common than a genuinely out-of-range beer. This matters beyond bragging rights — BJCP competition judges score against these published ranges, so knowing your real ABV from both formulas above is worth doing before you fill out an entry form.

Homebrew law and label-tolerance trivia

US federal law lets adults brew beer at home tax-free and without a permit under 26 U.S.C. § 5053(e): up to 100 gallons per calendar year in a household with one adult, or 200 gallons with two or more adults, strictly for personal or family use and never for sale. Two hundred gallons is more generous than it sounds — about 2,130 twelve-ounce servings a year. Homebrewing has been legal in all 50 states since 2013, when Mississippi and Alabama became the last two to authorize it, though states can still layer on their own possession and transport rules. A piece of labeling trivia worth knowing: commercial breweries get a legal tolerance of 0.3 percentage points either side of the ABV printed on the label under 27 CFR 7.65, so a beer labeled 5.0% can lawfully test anywhere from 4.7% to 5.3%. Your homebrew faces no labeling requirement at all — but the moment money changes hands the exemption evaporates, and selling even a single pint requires a TTB Brewer’s Notice plus state licensing. Brew, measure, share with friends over 21, and keep the ledger under 200 gallons.