How to Calculate ABV
Alcohol by volume comes straight from two hydrometer readings: the original gravity before fermentation and the final gravity after. As yeast converts sugar to alcohol, the liquid's density drops, and that drop maps directly to alcohol content. The standard homebrew formula multiplies the gravity difference by 131.25.
Original and final gravity
Take your OG reading when you pitch the yeast and your FG when fermentation finishes and the reading holds steady for a few days. A beer starting at 1.050 and finishing at 1.010 works out to about 5.25% ABV. Accurate, temperature-corrected readings matter — a sloppy hydrometer reading throws off the whole number.
Attenuation tells the story
Apparent attenuation is the percentage of available sugar the yeast consumed. Most clean ales land around 70-80%. Low attenuation means a sweeter, fuller beer or a stuck fermentation; very high means a dry, thin one. It's a quick health check on your yeast.
Frequently asked questions
- How do I calculate ABV?
- Subtract final gravity from original gravity and multiply by 131.25. For example, (1.050 - 1.010) x 131.25 is about 5.25%.
- When do I take gravity readings?
- OG before pitching yeast, FG when the reading is stable for a few days at the end of fermentation.
- What is attenuation?
- The share of sugar the yeast fermented — typically 70-80% for a clean beer.