Fermentation Attenuation Calculator
Attenuation measures how much of the fermentable sugar a yeast has converted, read from the fall in specific gravity. This calculator takes your original gravity and final gravity and returns the apparent attenuation, the real attenuation corrected for alcohol, and an estimate of alcohol by volume. Enter gravities in the usual decimal form such as 1.050 and 1.010. Use the result to judge whether fermentation is complete and how dry your finished drink will be.
Attenuation formula
apparent attenuation % = (OG - FG) / (OG - 1) * 100
real attenuation % = apparent attenuation * real factor
estimated ABV % = (OG - FG) * 131.25
gravity points dropped = (OG - FG) * 1000
OG and FG are decimal gravities such as 1.050. Apparent attenuation uses the raw gravities; real attenuation applies the standard 0.81 correction for alcohol. The ABV estimate uses the common 131.25 factor.
Worked example
- OG 1.050 to FG 1.010.
- Apparent attenuation = (0.050 - 0.010) / (0.050) times 100 = 80 percent.
- Real attenuation = 80 times 0.81 = 64.8 percent.
- Estimated ABV = 0.040 times 131.25 = about 5.25 percent.
- The gravity dropped 40 points (0.040 times 1000).
Attenuation: frequently asked questions
What is apparent attenuation?
Apparent attenuation is the percentage drop in specific gravity during fermentation: (OG minus FG) divided by (OG minus 1), times 100, using gravities such as 1.050 and 1.010. It is called apparent because the hydrometer reading is skewed by the alcohol produced, which is lighter than water.
What is real attenuation?
Real attenuation corrects for the presence of alcohol to reflect the true fraction of sugar fermented. A common correction multiplies apparent attenuation by about 0.81, the standard factor used in brewing because alcohol lowers the gravity reading. This calculator reports both apparent and real attenuation.
How is alcohol by volume estimated?
A widely used estimate is ABV = (OG minus FG) times 131.25, with OG and FG as decimal gravities like 1.050. For 1.050 down to 1.010 that gives (0.040) times 131.25 = about 5.25 percent ABV. It is an approximation; lab methods are more precise.
What attenuation should I expect?
Most ale and lager yeasts reach apparent attenuation of roughly 70 to 80 percent, though it varies by strain, recipe and process. Lower attenuation leaves more residual sweetness and body; higher attenuation produces a drier, more alcoholic result. Yeast suppliers publish typical ranges for each strain.
Official sources
- Alcohol and Tobacco Tax and Trade Bureau: beer specific gravity and alcohol content references.
- The attenuation and ABV relations are standard brewing formulas applied to your gravity readings.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.