# Really trick ABV claculation - Help!

### Help Support Homebrew Talk:

#### TequilaMockingbird

##### Active Member
Hi all,

Wonder if anyone can help me, as I am really not sure how to do the maths on this. I have finally kicked off a recipe I have wanted to do for ages. A chocolate porter with rum, raisins and banana.

My hyrdometer readings give the indication that the final ABV of the base beer will be 3.3% abv.

To the final beer I am naturally adding rum, and cannot work out the final ABV based on the following amounts and strengths. Has anyone any idea how I would go about doing this?

23 litres beer at 3.3%abv
+
20cl Rum at 37.5% vol

Bit tricky, I know, hope someone can help,

#### BobbiLynn

##### Well-Known Member
Hi all,

Wonder if anyone can help me, as I am really not sure how to do the maths on this. I have finally kicked off a recipe I have wanted to do for ages. A chocolate porter with rum, raisins and banana.

My hyrdometer readings give the indication that the final ABV of the base beer will be 3.3% abv.

To the final beer I am naturally adding rum, and cannot work out the final ABV based on the following amounts and strengths. Has anyone any idea how I would go about doing this?

23 litres beer at 3.3%abv
+
20cl Rum at 37.5% vol

Bit tricky, I know, hope someone can help,

I think you can just move the decimal point to the left 2 spaces since 20 is close enough to 23 to get an estimate. So 3.3 + .375 = 3.675 ABV. This is what it would be if you had 20 litres of beers, since you have 23, will be slightly lower, but no lower than 3.6 ABV.

OP
T

Many thanks!

#### Seedly

##### Well-Known Member
Actually it wouldnt give you that much of a bump.

The beer has 75.9 cl of ethanol in 2300 cl of liquid (2300*.033).
The rum has 7.5 cl of ethanol in 20 cl of liquid (20*.375)

So the total is 83.4 cl of alcohol in 2320 cl of liquid, or 3.59% ABV.

#### BobbiLynn

##### Well-Known Member
So the total is 83.4 cl of alcohol in 2320 cl of liquid, or 3.59% ABV.
Okay, I submit to the real math behind it, maybe slightly lower than 3.6, but I'd always estimate a 3.59 up to 3.6.

#### Seedly

##### Well-Known Member
Okay, I submit to the real math behind it, maybe slightly lower than 3.6, but I'd always estimate a 3.59 up to 3.6.
Its not much variance in this example, but it could be a much bigger variance in an example where more liquor is added. I thought it might be nice to do the actual math for someone else to use as an example when the thread came up in a search later.