Calculator
Attenuation calculator
How far did my fermentation actually go?
Attenuation is the proportion of the available sugar that the yeast actually fermented, and it is the single most informative number about how a fermentation went. It tells you more than the final gravity alone, because the same final gravity means very different things depending on where the beer started.
Most ale yeasts land between 70% and 80% apparent attenuation. Well below that suggests a stall, an under-pitch, a mash run too warm, or simply that fermentation is not finished. Well above it suggests wild yeast or a diastatic strain that will keep going. Apparent attenuation is the figure normally quoted and it slightly overstates the truth, because alcohol is lighter than water and drags the hydrometer reading down with it.
Result
- Attenuation78.8%
- Real attenuation64%
- Original extract12.9°P
- Apparent extract2.8°P
Working
Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100
| Step | Expression | Value |
|---|---|---|
| Gravity drop | OG − FG | 0.0410 |
| Original gravity points | OG − 1 | 0.0520 |
| Apparent attenuation | drop ÷ points × 100 | 78.8% |
| Real extract | 0.1808 × OE + 0.8192 × AE | 4.63 °P |
What it means
Most ale strains land at 70–80% apparent attenuation. Saison strains routinely exceed 85%, and English strains often stop below 72%, which is where their characteristic residual sweetness comes from.
Worked example
The example above starts from these values. It is rendered on the server, so the formula, assumptions and limitations are all present whether or not you change anything.
- Result78.8%
- Real attenuation64%
- Original extract12.9°P
- Apparent extract2.8°P
Inputs
| Original gravity | 1.052 |
|---|---|
| Final gravity | 1.011 |
How it is calculated
Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100
What each term means
| OG | Original gravity |
|---|---|
| FG | Final gravity |
Step by step
| Step | Expression | Value |
|---|---|---|
| Gravity drop | OG − FG | 0.0410 |
| Original gravity points | OG − 1 | 0.0520 |
| Apparent attenuation | drop ÷ points × 100 | 78.8% |
| Real extract | 0.1808 × OE + 0.8192 × AE | 4.63 °P |
What this assumes
- Fermentation is complete and final gravity is stable across two readings 48 hours apart.
Where it stops being reliable
- Apparent attenuation reads high because alcohol is less dense than water, so a hydrometer in finished beer under-reads. Real attenuation corrects for that and is always the lower figure.
- Attenuation is not purely a property of the yeast — mash temperature determines how fermentable the wort was in the first place.
What the number means
Most ale strains land at 70–80% apparent attenuation. Saison strains routinely exceed 85%, and English strains often stop below 72%, which is where their characteristic residual sweetness comes from.
Mathematical identity. Follows from the definitions of its terms. There is no approximation here and nothing to cite — if the inputs are right, the output is exact.