ABV & Attenuation Calculator
Two hydrometer readings are all it takes. Enter your original gravity (before fermentation) and final gravity (when it's done) to get alcohol by volume, how far the yeast attenuated, and whether your brew looks finished. Works for beer, cider, mead and wine. Free, no sign-up, all in your browser.
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Brewing gravity questions, answered
How is ABV calculated from gravity?
The widely-used standard formula is ABV ≈ (OG − FG) × 131.25. So a beer going from 1.050 to 1.010 is about (0.040 × 131.25) ≈ 5.3% ABV. It's an approximation that's accurate for most session and standard-strength brews; for big beers, wines and meads the alternate formula tracks reality a little better at higher alcohol levels.
What is apparent attenuation?
It's how much of the available sugar the yeast converted, as a percentage: (OG − FG) ÷ (OG − 1) × 100. Most ale yeasts finish somewhere around 70–80% apparent attenuation. Low attenuation can mean a sweeter, fuller beer by design — or a fermentation that stalled early.
How do I know fermentation has actually finished?
The reliable test is a stable gravity: take a reading, wait two to three days, and take another. If the number hasn't moved, fermentation is done and it's safe to bottle or keg. A single reading can't tell you that on its own — this calculator flags when your FG still looks high for the OG, which is a hint to wait and re-check.
Do I need to correct for temperature?
Yes, slightly. Hydrometers are usually calibrated to 20°C, so a hot sample reads low and a cold one reads high. For best accuracy, cool (or warm) your sample toward calibration temperature, or apply your hydrometer's temperature-correction chart before entering the figures here.