Alcohol and attenuation
Strength and attenuation from original and final gravity.
Two numbers — original and final gravity — give the strength of the beer and how completely the wort fermented. These are the main figures used to check a result against the recipe.
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How alcohol is calculated
The formula links the drop in gravity to the alcohol mass fraction and then converts it to a volume fraction, correcting for the density of the finished beer. The results come within tenths of a percent of a laboratory analysis — good enough for a home brewery.
ABW = 76.08·(OG − FG) / (1.775 − OG) ABV = ABW · FG / 0.794
Apparent and real attenuation
Apparent attenuation is calculated from the simple drop in gravity: (OG − FG) / (OG − 1). But alcohol is lighter than water, so it “pulls down” the gravity of the finished beer, and the apparent figure is always about 10–15 % higher than the real one.
In practice the apparent figure matters more: it is what you compare against a yeast strain’s stated attenuation, and what recipes report. Real attenuation is useful when you need to understand how much extract actually remains and whether the beer has dried out too far.
Typical values
| Style | Gravity and strength |
|---|---|
| Session pale ale | 1.040 → 1.008, about 4.2 % ABV |
| American IPA | 1.060 → 1.012, about 6.3 % ABV |
| Stout | 1.055 → 1.014, about 5.4 % ABV |
| Belgian strong ale | 1.080 → 1.012, about 9.0 % ABV |
What affects the result
- Yeast strain: stated attenuation ranges from 65 % for English strains to 85 % for some Belgian ones.
- Mash temperature: 63–65 °C yields more fermentable sugars, 67–69 °C more dextrins and a less dry beer.
- Grain bill: a share of crystal and roasted malt lowers attenuation.
- Mash thickness and aeration: they affect how completely the beer ferments more than people usually think.