Final Gravity Calculator
Final gravity (FG) is the specific gravity of beer after fermentation is complete, predicted from the original gravity and the yeast strain's apparent attenuation. The formula is FG = 1 + (OG − 1) × (1 − attenuation / 100). Mash temperature, grain bill composition and pitch rate all shift the result, typically within a ±5 percentage point window around the yeast's published figure.
Yeast strains publish an apparent attenuation range. That, plus your original gravity, is enough to know roughly where the beer will finish.
The prediction
Apparent attenuation is the proportion of the gravity points the yeast will remove, so:
FG = 1 + (OG − 1) × (1 − attenuation)
A 1.055 wort with a 75% attenuating yeast is predicted to finish at 1.014. The ±5% range shown alongside is not padding — it is roughly the spread you should actually expect between batches.
What moves the number
- Mash temperature. The biggest lever you control. A 62 °C mash produces a far more fermentable wort than a 70 °C one, and the same yeast will attenuate several percent further.
- Crystal malt and adjuncts. Crystal malt sugars are largely unfermentable, so a grain bill with 10% crystal finishes higher than the yeast’s rating suggests. Simple sugars go the other way and dry the beer out.
- Pitch rate and health. Underpitching, or pitching into cold wort, routinely costs several points of attenuation.
- Fermentation temperature. Warmer generally attenuates further, up to the point where the yeast starts making flavors you did not want.
Typical attenuation by yeast type
- English ale strains — 67–72%, leaving body and a higher finishing gravity
- American ale strains — 73–80%, clean and predictable
- Lager strains — 75–82%
- Belgian and saison strains — 78–90%, and some saison yeasts will take a wort under 1.002 given time
The yeast attenuation chart lists published figures for the strains in Brewgravity’s ingredient database.
Checking it afterwards
Once the beer is finished, put the actual OG and FG into the ABV calculator to see what you really got. If attenuation came in well below the yeast’s rating, the fermentation stopped early rather than the prediction being wrong.
Final Gravity Calculator — questions
My beer finished higher than predicted. Why?
Most often a high mash temperature, a lot of crystal malt, or an underpitched or cold fermentation. Warming the fermenter a couple of degrees and rousing the yeast will often restart it if it has stalled.
How do I know when fermentation is actually finished?
Two identical gravity readings taken three days apart. Airlock activity tells you nothing useful — it stops long before the yeast does, and it starts again if the temperature changes.
Where do I find my yeast's attenuation?
The manufacturer publishes it on the packet or the strain page, usually as a range. Brewgravity has it built in for the strains in its database, so choosing a yeast fills the figure in for you.
FG prediction that follows the recipe
Pick a yeast in Brewgravity and the predicted final gravity and ABV update instantly from its attenuation — no separate calculator needed.
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