Brew Gravity Calculator
Understanding Brew Gravity and Fermentation
Brewing beer or wine involves a fascinating transformation of sugars into alcohol and carbon dioxide, primarily driven by yeast. Measuring specific gravity is a fundamental practice for brewers to monitor this process and predict the final characteristics of their beverage. This calculator helps you understand key metrics derived from your gravity readings.
What is Specific Gravity?
Specific gravity is a measure of the density of a liquid compared to the density of water. In brewing, it indicates the amount of dissolved sugars and other solids in your wort (unfermented beer) or must (unfermented wine). Pure water has a specific gravity of 1.000.
Original Gravity (OG)
Original Gravity (OG) is the specific gravity of your wort or must before fermentation begins. It's a crucial measurement because it tells you the initial sugar concentration available for the yeast to convert into alcohol. A higher OG generally means more fermentable sugars, leading to a potentially higher alcohol content in the final product.
Final Gravity (FG)
Final Gravity (FG) is the specific gravity of your beer or wine after fermentation is complete. As yeast consumes sugars and produces alcohol, the density of the liquid decreases. A lower FG indicates that more sugars have been fermented. When the FG stabilizes over a few days, it's a good sign that fermentation has finished.
Alcohol By Volume (ABV)
Alcohol By Volume (ABV) is the percentage of alcohol in your finished beverage. It's calculated directly from the difference between your Original Gravity and Final Gravity. The greater the drop in specific gravity, the more sugar was converted, and thus, the higher the ABV. This calculator uses a widely accepted formula for homebrewing to estimate ABV.
Apparent Attenuation (AA)
Apparent Attenuation (AA) is a percentage that represents how much of the original sugar content *appears* to have been fermented by the yeast. It's called "apparent" because alcohol is less dense than water, which affects the final gravity reading, making it seem like more sugar was consumed than actually was. A higher AA indicates a drier beer.
Real Attenuation (RA)
Real Attenuation (RA) provides a more accurate measure of the actual percentage of fermentable sugars converted by the yeast. It accounts for the density-lowering effect of alcohol, giving a truer picture of the yeast's efficiency in consuming sugars. While Apparent Attenuation is commonly cited, Real Attenuation offers a deeper insight into the fermentation process.
How to Use This Calculator
Simply enter your measured Original Gravity (OG) and Final Gravity (FG) readings into the respective fields. The calculator will instantly provide you with the estimated Alcohol By Volume (ABV), Apparent Attenuation (AA), and Real Attenuation (RA) for your brew. These metrics are invaluable for consistency, recipe development, and understanding your fermentation performance.
Example Calculation:
Let's say you brewed an IPA:
- Original Gravity (OG): 1.065
- Final Gravity (FG): 1.012
Using the calculator, you would find:
- ABV: (1.065 – 1.012) * 131.25 = 6.90%
- Apparent Attenuation: ((1.065 – 1.012) / (1.065 – 1)) * 100 = 81.54%
- Real Attenuation: (Requires intermediate Real Final Gravity calculation) = Approximately 65.0%
These results tell you that your IPA is a moderately strong beer with a high degree of sugar conversion, indicating a relatively dry finish.