Attenuation refers to the amount of sugar that is converted to alcohol and CO2 during fermentation. If a given wort contains 20% sugar pre-fermentation, and 4% sugar post-fermentation, then the attenuation is 80%, since 80% of the sugars were converted. Homebrewers tend not to measure percent of sugar directly. Instead, we use specific gravity as an indicator of how much sugar is in the wort (speciific gravity is easy to measure with inexpensive instruments). Unfortunately, since alcohol is less dense than water, specific gravity is really not an accurate indicator of sugar content once fermentation has started. (Indeed, the higher the OG, the less accurate the FG is in measuring sugar content, since there tends to be more alcohol in a higher gravity beer). Nevertheless, we use that number and calculate what's called the apparent attenuation rather than the actual attenuation. The various formulae that we all use to calculate ABV ( (OG - FG)/131, for example) have a fudge factor built in to give us a number that's reasonably close to reality for 'normal' gravity beers. While not strictly accurate, it's meaningful enough for our purposes and, as long as everybody uses the same errors consistently, it's good enough to tell us how these beers compare, ABV-wise. The simple formula for Apparent Attenuation is (OG - FG)/(OG - 1). (even simpler, just drop the 1 before the decimal point and use (OG - FG)/OG ). So a beer with an OG of 1.050 and an FG of 1.010 is said to have an apparent attenuation of (50 - 10)/(50), or 80% (you can do this in your head easily enough -- 10 is 80% less than 50). And there you have it!
Mike did a great job explain things. My response would have been (and is) the way I calculate attenuation (apparent attenuation):http://pint.com.au/calculators/alcohol/ Cheers!
