ABV Calculator — homebrew alcohol from OG/FG
Calculate alcohol by volume and attenuation from original and final gravity, with a Plato ⇄ SG helper — for homebrewing, in your browser.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
Enter original and final gravity (SG) to estimate alcohol by volume and apparent attenuation.
ABV ≈ (OG − FG) × 131.25 — a common approximation; real ABV varies with wort composition. Attenuation = (OG − FG) ⁄ (OG − 1) × 100. All results are estimates.
⚠ Legal note: in Japan, fermenting beverages above 1% ABV at home is restricted by the Liquor Tax Act (酒税法). This tool is for educational use and for homebrewing where it is legal in your jurisdiction.
About this tool
Homebrewers track fermentation with two hydrometer readings, and this ABV calculator turns them into answers: enter your original gravity (OG) and final gravity (FG) to get alcohol by volume via the standard formula (OG − FG) × 131.25, plus apparent attenuation so you can judge how completely the yeast finished. A built-in converter switches between specific gravity and degrees Plato for brewers who work in either scale. It covers beer, mead, cider and wine ranges. All calculations happen locally in your browser — your readings are never sent anywhere.
Frequently asked questions
How do I take hydrometer readings, and how accurate is the formula?
Float the hydrometer in a sample jar of wort or must, spin it to shed bubbles, and read at eye level at the calibration temperature (usually 20°C) — take OG before pitching yeast and FG once the reading is stable for two or three days. The (OG − FG) × 131.25 formula is a linear approximation: reliable within about 0.1–0.2% ABV for ordinary-strength beers, but it drifts for very strong brews above roughly 1.070 OG, where more elaborate formulas do better. For labelling precision, only lab analysis is definitive.
What does attenuation tell me?
Apparent attenuation — (OG − FG) ÷ (OG − 1) as a percentage — measures how much of the fermentable sugar the yeast consumed. Most ale and lager yeasts land between 70% and 85%, per the strain's published range. A figure far below that suggests a stuck fermentation (temperature drop, stressed yeast), leaving the beer sweet and potentially leading to overcarbonated bottles if you package too early. A figure near or above the top of the range means a very dry finish. Compare your number against the yeast datasheet before packaging.
Is homebrewing legal, and are my readings private?
It depends where you live. In Japan, the Liquor Tax Law (shuzeihō) prohibits making beverages of 1% ABV or more without a licence — home fermentation of beer, wine or mead above that threshold is not permitted, which is why Japanese kits are formulated to stay under 1%. Many other countries, including the US and UK, allow personal homebrewing within limits. Always check your local law. The calculator itself just does math: your gravity readings are processed in your browser and never transmitted.
Do I need to temperature-correct a hydrometer reading?
Yes, if you want the third decimal to mean anything. Hydrometers are calibrated at a specific temperature — usually 20 °C, sometimes 15.6 °C — and warm liquid is less dense, so a hot sample reads low. Around 10 °C above calibration costs roughly 0.001 SG, which shifts a calculated ABV by about 0.13%. Either cool the sample to the calibration temperature before reading, or apply your hydrometer's correction table; guessing is what makes two people disagree about the same batch.
Can I use a refractometer instead?
Before fermentation, yes — a refractometer needs only a drop or two and reads original gravity accurately. After fermentation it goes wrong, because alcohol bends light differently from sugar and the instrument cannot tell them apart, so the final reading comes out high and the ABV low. Either convert the final reading with a refractometer correction formula, or take OG with the refractometer and FG with a hydrometer, which is what most brewers settle on.