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Home Brew Forums > Home Brewing Beer > Gluten Free Brewing > Calculating ABV
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Old 05-24-2010, 07:52 PM   #1
Androshen
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Default Calculating ABV

I am having problems calculating my ABV ...
I had been using the formula (OG - FG) X 105 = ABV but my friend (who are more experienced drinkers) all tell me that the ABV is considerably higher than what I have calculated.
I have since checked online and found the following formulas ... also for determining ABV from the SG readings:

(OG - SG) X 131

(OG - SG) / 7.36

(OG - SG) / .75

(OG - SG) X 129

(OG - SG) / 7.69

Why are there so many different formulas and which one is correct?

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Old 05-24-2010, 08:00 PM   #2
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well i subscribe to formulas 1 and 4. they are the same formula. the 131 and 129 are dependent on how high your OG was. there is a table out there somewhere that lists the constants you are supposed to use in that formula.

as far as which of these are actually correct i can't say. without actual lab testing its hard to say exactly what your ABV actually is. these formulas are just meant to get you into the ball park. obviously your old ball park isn't close enough, at least according to your friends.

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Old 05-24-2010, 08:29 PM   #3
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I'll say!! 131 and 129 are *a long way* from 105!!

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Old 05-24-2010, 09:06 PM   #4
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The one I've always used is 1.05 X (SG-FG)/FG/.79

You get the ABW before dividing by .79, and then the ABV when you do

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Old 05-24-2010, 10:33 PM   #5
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All of those formulas are similar (some return a percentage and some a number but that should be obvious) in that they assume that alcohol production is a linear function of the drop in apparent extract, which it is most certainly not. Therefore they are all right some of the time and wrong most of the time. Alcohol production is much closer to the drop in real extract, but we can't measure that directly with rudimentary techniques.

Note that I believe that .131 approximation comes from the home wine making literature were it would be used for a fairly narrow range of apparent extract reductions and is probably appropriate. Attenuation of beer ranges much more widely and is generally lower, I would guess than .129 approximation is an attempt to create an estimation more suitable to beer but who knows.

A much more robust estimation which involves first estimating real finishing extract is available in the brewing literature. Here is a list of those and other estimations published on HBD by George Fix. This will be more correct than anything above but I personally can't remember it and do it in my head like I can with x .131.

http://www.realbeer.com/spencer/attenuation.html

Any of these estimations are good enough to get you vaguely in the ballpark but then so are your nose and tongue.

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Old 08-24-2010, 02:17 AM   #6
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Is it possible to determine your ABV if you never took a hydrometer reading? In all my excitement of my first brew I never took a reading and dismissed taking a reading when siphoning into primary and then to secondary fermenter because I thought I had missed the boat. I think the floating 3-way hydrometer has intimidated me so much that I am avoiding trying to solve its complexity. Any thoughts? Am I too late to the party?

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Old 08-24-2010, 06:46 AM   #7
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I found a formula on the net that uses SG and Brix for finished wine. Not absolutely accurate, but close enough for comparison of personal production.

Tools needed, hydrometer and refractometer.

( brix x 4 ) + 1000, -SG. divide by 3.3 = ABV

Sample: loquat wine, sg 998, brix 7.5

So, 7.5x4=30, plus 1000= 1030, -998= 32, divide by 3.3= 9.7, so call it 10% Stabilize and add a bit more sugar, I like 10/10 wine- 10% sugar, 10% abv.

Original formula went through too many steps of dividing by ever smaller numbers, out to 5 or 6 decimal places. I don't think I need to know that the loquat wine above is actually 9.6969696969 abv. And how accurate are your instruments, at what temp? So I simplified, then tested several commercial samples of wine, compared to the label. As I recall, about +/- 1/2%. Close enough for a home brewer.

The potential alcohol as listed on the side of the hydrometer scale won't work for wines made from must, too much unfermentable junk in the wine to trust the OG.

For not-must concoctions, I use the change in the refractometer brix reading, before and after, divide by 2 to get abw, times 1.21 to get abv.

Hope this helps, feedback would be appreciated- try it on some commercial stuff?

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Old 10-20-2012, 03:07 PM   #8
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Quote:
Originally Posted by casebrew View Post
I found a formula on the net that uses SG and Brix for finished wine. Not absolutely accurate, but close enough for comparison of personal production.

Tools needed, hydrometer and refractometer.

( brix x 4 ) + 1000, -SG. divide by 3.3 = ABV

Sample: loquat wine, sg 998, brix 7.5

So, 7.5x4=30, plus 1000= 1030, -998= 32, divide by 3.3= 9.7, so call it 10% Stabilize and add a bit more sugar, I like 10/10 wine- 10% sugar, 10% abv.

Original formula went through too many steps of dividing by ever smaller numbers, out to 5 or 6 decimal places. I don't think I need to know that the loquat wine above is actually 9.6969696969 abv. And how accurate are your instruments, at what temp? So I simplified, then tested several commercial samples of wine, compared to the label. As I recall, about +/- 1/2%. Close enough for a home brewer.

The potential alcohol as listed on the side of the hydrometer scale won't work for wines made from must, too much unfermentable junk in the wine to trust the OG.

For not-must concoctions, I use the change in the refractometer brix reading, before and after, divide by 2 to get abw, times 1.21 to get abv.

Hope this helps, feedback would be appreciated- try it on some commercial stuff?
REALLY sorry to necro a thread but i've been really confused by this lately. and would rather that than start a duplicate
Here are my numbers for the batch I'll use as an example.
The OG was 1.045, and then i added sugar, bringing OG = 1.060
From 1.060 on the hydrometer, i can look to the balling (brix) scale and see that this gravity would give me a reading of 15, and a potential alcohol of just shy of 8%. The final gravity of this cider was 0.999.

Now, using the above brix equation gives me:
( 15 x 4 ) + 1000, - 999. divide by 3.3 = ABV = 18.49%

BUT using the hydrometer's (admittedly imprecise) scale of potential alcohol, i can only possibly have less than half that.

Going back to the usual calculation using the gravities,

(OG-FG) x (F) = ABV%
As OP Said, there are a bunch of F's i've found, but this seems to be the most reputable. If i use the F given by that site, I end up with the following:

(1060-999) x (0.132) = 8.05 which seems like it makes sense.

I guess my question is, why doesnt the brix equation work, and should i be using these F's every time i do a calculation instead of choosing somebody's constant since everyone seems to use something different? If so, I'll definitely be copying that table into my notebook.

edit: also, it says "
f is the factor connecting the change in gravity to alcoholic strength. The value of 'f' is not constant because the yield of alcohol is not constant for all fermentations. In lower strength beers, more of the 'sugars' available for fermentation are consumed in yeast reproduction than in producing alcohol."

So, my question then becomes wouldnt the factor change also depending on how much yeast i pitch into the must, doesnt adding more yeast cells off the bat reduce the need (at least a little bit) for the yeast to reproduce, or am i just really really overthinking this.
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Old 10-20-2012, 06:13 PM   #9
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OK so I don't calculate by hand. I use an online calculator, and it's never done me wrong. Obviously our data are far from perfect, but my paper and pencil aren't going to help that.

I can tell you that 1.060 isn't going to give you 18+% ever. That's gonna need some serious gravity.

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