Wed Dec 14, 2011 6:31 am
As long as the amounts of alcohol being added are small you can do it by assuming consevation of volume e.g. if you have 5 gal of beer at 5% abv that contains 0.25 gal of alcohol. If you add 1 quart of 80 proof spirits you would be adding 0.4 quart (0.1 gal) of ethanol for a total of 0.35 gal of ethanol in 5.25 gal of fortified beer. ABV = 100*0.35/5.1 = 6.9%. Accurate calculations are much more detailed than this and require that you can convert between SG and Plato, have "proofing tables" or data on the density of ethanol - water solutions etc. At the levels of this example the extra effort buys minimal increase in accuracy - much less than the uncertainty in your knowledge of the ABV of your beer unless you sent it out for analysis.
