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Old 08-10-2010, 11:38 PM   #1
BVilleggiante
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Hello,

Is there a way to calculate the end specific gravity of your beer? I know you take a reading of the OG before fermentation, but how do you know what the specific gravity should be before it's done fermenting? Also, is there a way to predict the alcohol content of your brew?

Sorry, very new to this. Still reading and haven't even done my first brew yet. I've seen the things above mentioned, but not how to calculate them.

 
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Old 08-10-2010, 11:41 PM   #2
ChshreCat
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I punch all the ingredients into beersmith and it gives me an estimate. You can do the same on the beer calculus website for free. There are a lot of variables so it's just a ballpark estimate, but it's usually close. There's a few different formulas you can use to figure out your ABV based on the OG and SG. I just let beersmith do the math for me.

 
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Old 08-10-2010, 11:48 PM   #3
BVilleggiante
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I think I'm going to have to look at that software. I've heard pretty good things.

 
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Old 08-10-2010, 11:55 PM   #4
Bmorebrew
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It depends on how fermentable your sugars are. For example if you mash too high and end up with more unfermentable sugars, your FG will be higher than if you mash at a lower temperature (thus ending up with more fermentable sugars).

 
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Old 08-11-2010, 12:51 AM   #5
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I agree with the Cat... Beersmith is a great program and has lots of included calculators.
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Old 08-11-2010, 01:02 AM   #6
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1) Typically FG is OG/4. So a 1.040 OG will end at about 1.010. Note that you do the math on the "40" part of the number, not "1.040".

2) ABV = OG-FG / 7.6. So, 40-10 / 7.6 = 3.94%
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Old 08-11-2010, 02:59 AM   #7
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Thanks Passpawn. Even if I get beersmith it's good to understand the math behind it.



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Originally Posted by passedpawn View Post
1) Typically FG is OG/4. So a 1.040 OG will end at about 1.010. Note that you do the math on the "40" part of the number, not "1.040".

2) ABV = OG-FG / 7.6. So, 40-10 / 7.6 = 3.94%

 
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Old 08-11-2010, 03:26 AM   #8
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Let me correct passedpawn's formula. It should be ABV = (OG-FG) / 7.6. So, (40-10) / 7.6 = 3.94%. The other variation is (OG - FG) * 131, so (1.040 - 1.010) * 131 = 3.93%.
Same formula with rounding differences.
If you know what your OG and FG are, you can calculate the ABV pretty accurately.
The problem comes with predicting what the OG and FG will be.
It is relatively simple to predict the OG given the correct information, but in my experience, some of the extract potentials in Beersmith do not match the data sheets produced by the maltsters. In my case, this results in about a 5% reduction in the OG.
What is far more important is that the prediction of the FG is about 20% higher than I typically achieve. This makes (for me) the Beersmith predictions for FG and ABV to be totally useless.

-a.
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Old 08-11-2010, 03:52 AM   #9
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Thanks Alf. This too is good info and thanks for the info and beersmith. That makes me leery of it.

 
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Old 08-11-2010, 04:03 AM   #10
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Quote:
Originally Posted by ajf View Post
Let me correct passedpawn's formula. It should be ABV = (OG-FG) / 7.6. So, (40-10) / 7.6 = 3.94%. The other variation is (OG - FG) * 131, so (1.040 - 1.010) * 131 = 3.93%.
Same formula with rounding differences.
If you know what your OG and FG are, you can calculate the ABV pretty accurately.
The problem comes with predicting what the OG and FG will be.
It is relatively simple to predict the OG given the correct information, but in my experience, some of the extract potentials in Beersmith do not match the data sheets produced by the maltsters. In my case, this results in about a 5% reduction in the OG.
What is far more important is that the prediction of the FG is about 20% higher than I typically achieve. This makes (for me) the Beersmith predictions for FG and ABV to be totally useless.

-a.
Well, yea, predictions of any kind are useless once you have measured the actual numbers. Until then, they are very useful in designing a beer (IMO). Especially for a new brewer.

I don't think anyone should avoid using beersmith, promash, or any other tool. Brewing is better with them than without them, regardless of some very minor flaws.
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