How would one calculate the ABV of a beer if distilled spirits were added? For instance, when making a beer recipe that called for adding bourbon as a flavoring agent. If you started with, say, 5 US gallons of beer at 5% ABV and you added 1 quart of "80 proof" spirits, how would one calculate the resulting ABV?

  • US gallons or imperial gallons?
    – Pharap
    Sep 7, 2014 at 0:32
  • Added "US" to question.
    – Dale
    Sep 8, 2014 at 22:29

1 Answer 1


This seems like a simple solution dilution problem. Take

5 gallons = 20 quarts => 20 quarts * 5% ABV = 1 quart alcohol.

Then take your 1 quart of "80 proof" (40% ABV), and we get

1 quart * 0.4 = 0.4 quarts alcohol

So we have a total volume of 21 quarts (beer plus spirits) and a total of 1.4 quarts alcohol, thus

1.4 / 21 = 6.67% ABV

Unless I'm missing something about ABV percentage, 6.67% should be the final answer.

In general, if we have (making sure we are using consistent units)

O - the original volume, in quarts
A - the volume of spirits to add, in quarts
s - the % ABV of the original volume
p - the % ABV of the spirits

Then we can use the general formula below

((O * s) + (A * p)) / (O + A)
  • Is that a US gallon or an imperial gallon?
    – Pharap
    Sep 5, 2014 at 23:48
  • US gallon, but the idea is the same, get the total alcohol in each fluid, and combine
    – CDspace
    Sep 6, 2014 at 0:33
  • Just clarifying since these differences can cause issues sometimes. (For instance 40% ABV is 80 proof in the US but 40% ABV in the UK is 70 proof.) I should have realised though that it's a matter of ratio the units in use are slightly irrelevant.
    – Pharap
    Sep 7, 2014 at 0:53
  • @pharap Indeed, this type of problem is a basic homework problem in university physics 1. The units don't matter as much as the method.
    – CDspace
    Sep 7, 2014 at 0:55
  • 1
    @dale that would probably work better as a separate question. I'm not really sure how that might be done
    – CDspace
    Sep 13, 2014 at 16:36

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