Purpose: To determine the alcohol percentage by volume or weight
Results: *** Waiting for results ***
The amount of alcohol in beer depends upon the amount of sugar consumed by the yeast, which is the difference between the specific gravity at the beginning (original gravity; OG) and end of fermentation (final gravity; FG). There several different formulas available for calculating alcohol that vary slightly from each other. The Wikipedia Alcohol by volume page has two simple formulas for calculating alcohol by volume. The minor differences between the formulas are attributed to rounding and temperature variations.
There are also high gravity formulas that may be more accurate for high gravity beers. Formula #3 from the Wikipedia ABV page is for alcohol concentrations above 6%. Formula #4 is based on real extract calculations that make adjustments based on the effects of alcohol on the gravity readings.
The results from these formulas are all pretty close. For homebrewers, the differences are trivial.
Be sure to use corrected specific gravity values if your specific gravity measurements are taken at a temperature higher than 60F (15.5C; the standard temperature calibration of most hydrometers).
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