Calculating, Specific Gravity and ABV

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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.
 
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.
 
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).
 
I agree with the Cat... Beersmith is a great program and has lots of included calculators.
 
Thanks Passpawn. Even if I get beersmith it's good to understand the math behind it.



passedpawn said:
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%
Click to expand...
 
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.
 
ajf said:
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.
Click to expand...

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.
 
I agree that Beersmith is a useful tool, and the grain database can easily be updated to match the specifications, but the FG prediction can be very inaccurate. As the OP was asking about predicting the FG and the ABV (which requires the FG), I thought it was good to point out this limitation.
I am not aware of any software that can make accurate FG predictions.

-a.
 
If you have an OG, and know the attenuation % range of the yeast.
Can't you just multiply and subtract?
If the yeast has an attenuation range of 70 to 75%
OG of 1.050
FG 1.015 to 1.0125
ABV 4.65 to 4.99
 
To estimate the final ABV, I just take the OG and drop the ‘1’. For example, a 1.055 OG will give a 5.5% beer. A 1.062 OG will give 6.2%, etc. If you do the math assuming 75% attenuation, you’ll see it’s very close.

Let’s face it, we’re not chemists or commercial brewers. How accurate do you really need to be?
 
Some of us are chemists.
If you have an OG, and know the attenuation % range of the yeast.
Can't you just multiply and subtract?
If the yeast has an attenuation range of 70 to 75%
OG of 1.050
FG 1.015 to 1.0125
ABV 4.65 to 4.99
Click to expand...

That would assume then that 100% of your sugars are fermentable, but again that depends on your mash temperature. If you're only looking for a rough estimate, you can get away with it, but I would drop a significant figure from how ABV is reportedand use only integers. Instead of saying 4.6, say 5%.
 
Bmorebrew said:
Some of us are chemists.


That would assume then that 100% of your sugars are fermentable, but again that depends on your mash temperature. If you're only looking for a rough estimate, you can get away with it, but I would drop a significant figure from how ABV is reportedand use only integers. Instead of saying 4.6, say 5%.
Click to expand...

I don't go around saying a beer is 4.65 or 4.99% ABV
I just posted the #'s that way.
Rough estimate was what I was shooting for
 
In that case I think you'd be fine - any uncertainty built into the fermentability of the wort and attenuation of the yeast should easily be scrubbed by the time you present a rough estimate.
 

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