Beer Forum

I've made a Chocolate Oatmeal Rum Stout & a Vanilla Bourbon Porter. How the heck do I calculate the ABV after adding amounts of liquor?

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.

Great description! Thanks for the help! After reading a few different things, I decided to use this formula:

(X*ABV) + (Y*ABV2) / (X+Y) = New ABV%

X = Qty of beer
ABV = ABV% of beer
Y = Qty of spirits being added
ABV2 = ABV% of spirits

Note: Proof of Liquor/2 = ABV%