Attenuation Calculator
Enter original and final gravity to get apparent and real attenuation with ABV, or flip the mode to predict the final gravity a yeast at a known attenuation will deliver.
Results
- Apparent attenuation
- 76%
- Predicted final gravity
- 1.012 SG
- Alcohol by volume
- 4.99%
- Gravity drop
- 38 pts
How to use the attenuation calculator
- Measure original gravity before pitching the yeast.
- Measure final gravity once readings stop moving for days.
- Read apparent and real attenuation with the ABV.
- Or switch mode to predict FG from a yeast's rated attenuation.
Apparent attenuation is the share of dissolved extract the yeast consumed, read directly off the hydrometer: (OG - FG) / (OG - 1). Because alcohol is lighter than water, the hydrometer overstates it; real attenuation corrects that bias and lands about 20 percent lower. The reverse mode runs the same equation backwards to predict the final gravity a yeast's rated attenuation implies.
Both inputs start as hydrometer readings: fix any warm or cold sample with the hydrometer temperature correction tool, then read the full strength picture in the ABV calculator.
Frequently asked questions
Why is real attenuation lower than apparent?
Alcohol is less dense than water, so a fermented sample reads artificially low on a hydrometer. Real attenuation converts the reading to what the extract actually did, using a fixed correction factor; the gap is roughly 20 percent across the normal brewing range.
My beer stopped above the yeast's rated attenuation - what now?
First confirm it is truly stuck: take two readings two or three days apart at fermentation temperature. If unchanged, rouse the yeast, warm the fermenter a few degrees, or pitch fresh yeast. High mash temperatures, low pitch rates and old yeast are the usual causes rather than a bad yeast sheet.
Does higher attenuation always mean a better beer?
No - it is a style decision. A dry Irish stout or saison wants high attenuation, while an English bitter or oatmeal stout relies on unfermentable dextrins for body and balance. Match the yeast and mash temperature to the style instead of chasing the biggest number.