Alcohol By Volume (ABV) And Attenuation
The Original Gravity (OG) and Final Gravity (FG) readings input into the calculator are taken with a Hydrometer. If you take your Original Gravity (OG) and Final Gravity (FG) at similar temperatures somewhere around the hydrometer calibration point, there’s no need for a temperature correction. See the Hydrometer Temperature Correction Calculator for more information.
The Simple ABV formula is (OG-FG) * 131.25 = %ABV and works fine for most common beer types. However, this formula becomes less accurate with progressively higher alcohol content. The Most Accurate ABV formula takes into account that the fluid measured in the Final Gravity is no longer only water with dissolved solids since it now contains alcohol. So, if you are brewing strong beer, the difference becomes relevant.
Attenuation is the term used to describe the percent of sugar consumed by the yeast and turned into Alcohol, CO2 and other compounds. There are many factors which can have an effect on yeast attenuation such as mash temperature, fermentation temperature, yeast flocculation, and yeast health.
Apparent Attenuation and Real Attenuation
There are two types of Attenuation: Apparent Attenuation (AA) and Real Attenuation (RA). If a yeast provider only uses the word Attenuation, you can fairly safely assume they mean Apparent Attenuation.
Apparent Attenuation is the Attenuation number given by most yeast providers to convey expected Final Gravity to the brewer. This is often given as a percent range and is a close approximation to the percent change in gravity.
Apparent Attenuation is different from Real Attenuation. Real Attenuation (just as with the Most Accurate ABV formula) takes into account that the Final Gravity is no longer only water with dissolved solids and now contains alcohol. Real Attenuation can be calculated or measured by laboratory analysis, but generally is a number only used by large breweries.
Note: The “Most Accurate ABV” formula used in this calculator is based on Equation 16 from the Hall paper, not Equation 17 which uses a simplified conversion for extract to gravity and will produce an inflated result.