Yeast Attenuation Calculator

The yeast attenuation calculator determines how effectively yeast consumed the sugars in a batch of beer or wine, from gravity readings taken with a hydrometer or refractometer. It produces apparent attenuation, real attenuation, and alcohol by volume using both simple and advanced formulas.

Original Gravity (OG), the reading taken before fermentation begins, typically falls between 1.020 and 1.130 for most beer styles. Final Gravity (FG), the reading taken once fermentation is complete, must be lower than the original gravity and typically falls between 1.000 and 1.030. Both accept decimal values such as 1.050 or fractional notation such as 1050/1000.

Apparent attenuation is the gravity-based estimate that yeast manufacturers commonly reference on a packet or product page, so it is the figure to compare against a strain's published range. Real attenuation is always lower than apparent attenuation, because it corrects for the alcohol already present in the finished beer, which a hydrometer reads as if it were still unfermented sugar. Simple ABV and the most accurate ABV are reported side by side; the most accurate formula, based on Hall Equation 16, accounts for alcohol density and produces better results for higher-gravity beers, where the simple formula's error grows.

Settings can show the step-by-step derivation of each result, including the Plato conversion, the attenuation formulas, and the ABV equations with the entered values substituted in.

The apparent attenuation formula is AA = (OG - FG) / (OG - 1) x 100. Real attenuation uses the Plato scale: convert both gravities to degrees Plato, compute real extract as RE = 0.1808 x OGp + 0.8192 x FGp, then RA = (OGp - RE) / OGp x 100. The simple ABV formula is (OG - FG) x 131.25. The most accurate ABV uses Hall Equation 16: ABV = (76.08 x (OGp - FGp) / (1.775 - OGp)) x (FG / 0.794).

A pale ale reads 1.050 original gravity and finishes at 1.010 final gravity. Apparent attenuation works out to (1.050 - 1.010) / (1.050 - 1) x 100, or 0.040 / 0.050 x 100, which is 80 percent. Simple ABV comes out to (1.050 - 1.010) x 131.25, or 0.040 x 131.25, which is 5.25 percent alcohol by volume. Real attenuation would land somewhat below 80 percent for the same readings, since the Plato-based correction accounts for alcohol the hydrometer mistakes for sugar.

Evaluate yeast performance across a wide range of brewing scenarios. Track fermentation health, compare yeast strains, and troubleshoot stuck fermentations with precise attenuation measurements.

• Homebrewing: Measure how completely a yeast strain fermented a batch of pale ale with an OG of 1.055 and FG of 1.012 to verify the yeast performed within its expected 73 to 77 percent apparent attenuation range.
• Yeast Strain Comparison: Pitch two different yeast strains into identical wort samples and compare their apparent and real attenuation percentages to select the best strain for a recipe.
• Stuck Fermentation Diagnosis: Enter current gravity readings to determine whether fermentation stalled prematurely. An apparent attenuation well below the yeast strain's expected range indicates a potential problem with yeast health, temperature, or nutrient availability.
• Recipe Development: Predict the dryness and alcohol content of a new recipe by entering target OG and estimated FG based on the chosen yeast strain's published attenuation range.
• High-Gravity Brewing: Compare simple and most accurate ABV calculations for barleywines or imperial stouts where the OG exceeds 1.080. The difference between the two formulas becomes significant at higher alcohol levels.
• Quality Control: Track attenuation batch over batch for commercial or advanced homebrew operations. Consistent attenuation numbers indicate healthy yeast management and repeatable processes.
• Education: Learn the difference between apparent and real attenuation using the step-by-step formula display, and study how alcohol density affects the hydrometer reading.