Calculating Homebrew ABV from OG and FG
Alcohol by volume (ABV) is estimated from how much the wort's gravity drops between the start and end of fermentation. As yeast breaks sugar down into alcohol and carbon dioxide, the wort's density falls, and measuring that drop lets you back-calculate the alcohol content.
This calculator uses (OG-FG) x 131.25, the simplified formula most homebrewers rely on. It's within 0.2-0.5 percentage points of more precise formulas, which is plenty accurate for everyday homebrewing. High-gravity beers like imperial stouts can see a slightly larger gap, which is worth keeping in mind if you're brewing something big.
The apparent attenuation shown alongside the result tells you what share of fermentable sugar actually turned into alcohol. A healthy range is typically 65-80%. A lower number can mean fermentation isn't finished yet or that yeast activity was weaker than expected.
Frequently Asked Questions
OG (Original Gravity) is the wort's density before fermentation starts, and FG (Final Gravity) is what's left after fermentation ends. A bigger gap between the two means yeast converted more sugar into alcohol.
This calculator uses the simplified (OG-FG) x 131.25 formula, the most common one among homebrewers. It can differ from the true ABV by about 0.2-0.5 percentage points, which is accurate enough for typical homebrewing.
A higher-than-expected FG can mean fermentation hasn't finished. Check yeast health, fermentation temperature, and the fermentability of any added sugars, then wait a few more days and re-measure.