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Standard formula: ABV = (OG - FG) x 131.25. Take both readings at the same temperature (hydrometers calibrate at 20C) and degas the sample. A stable FG over 2-3 days means fermentation has finished: bottling early risks overcarbonation and burst bottles.
What The Homebrew ABV Calculator Works Out
- Alcohol by volume: the hydrometer reads the wort's density before yeast (original gravity) and after (final gravity), and the difference times 131.25 gives ABV. A beer starting at 1.050 and finishing at 1.010 is 5.25%. Most UK homebrew ales land between 4% and 6%; a 1.090 wine must finishing near 0.995 runs about 12.5%.
- Apparent attenuation: the percentage of the available sugar the yeast consumed, (OG - FG) divided by (OG - 1). Most ale yeasts land at 70% to 80%; an FG stuck at 1.020 from a 1.050 start says the yeast quit early, from cold, underpitching or unfermentable extract.
- Calories per pint: an estimate from alcohol plus residual sugar, roughly 180 to 230 kcal per pint for ordinary-strength beers. Alcohol carries about 7 kcal per gram, so most of a dry beer's calories are the alcohol itself.
- ABV formula (advanced): the standard 131.25 multiplier drifts optimistic above about 7% or 8%. The alternative formula, 76.08 x (OG - FG) / (1.775 - OG) x (FG / 0.794), tracks strong beers, ciders and wines better; the two agree to within a tenth of a percent on session-strength beer.
- Temperature correction (advanced): hydrometers are calibrated at 20C (older ones at 15.6C) and read low in warm samples. Enter the sample temperature and the calibration temperature and both gravities are corrected before the ABV is worked out. A 1.050 wort read at 30C is really about 1.052.
- Units per serving (advanced): UK alcohol units are millilitres times ABV divided by 1,000, so a 5.2% pint is 3 units. The NHS low-risk guideline is no more than 14 units a week, spread over three or more days.
- Whole batch (advanced): how many servings the batch yields at your chosen size, the total units in it and the litres of pure alcohol. A standard 23-litre (40-pint) kit at 5% holds about 115 units.
- Duty you did not pay (advanced): what the same beer would carry in UK alcohol duty if sold commercially, at the current rate per litre of pure alcohol for its strength band (GBP 21.78 for 3.5% to 8.4%, less for weaker beer and more above 8.5%), before VAT. Brewing beer, wine and cider at home for your own use needs no licence and pays no duty, and this figure is most of the hobby's famous economy. Distilling without a licence remains illegal. Please enjoy the results responsibly; 18+.