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Calculating %ABV Without the Initial Specific Gravity

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raguido

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Some Helpful Tips for Winemaking

One likes to know the final alcohol content after making a batch of wine. Typically, one can calculate this from the initial and final specific gravity. The initial specific gravity can be measured directly or from the Brix number from index of refraction measurement.

% ABV = (SGf – SGi)/0.0074, where SGf is the final specific gravity and SGi is the initial specific gravity.

For example, if the initial specific gravity was 1.110 and the final value was 0.996, the calculated alcohol content would be 15.4%. But what if you forgot to get the initial specific gravity? The specific gravity can be calculated from the Brix number only in the absence of alcohol. However, if one measures the Brix number AND the specific gravity after fermentation, the %ABV can be calculated from the following formula:

% ABV = (1.646*Brix) – (2.703*(145-145/SG)) – 1.794

The easiest way to do this routinely is to use an Excel spreadsheet. For example, in cell C1 enter the Brix value (=Brix). In cell C2, enter the SG (=SG). Then in cell B7, enter (copy and paste):

=1.646*C1-(2.703*(145-145/C2))-1.794

This will be the final %ABV. You can label the cells appropriately for use, such as “Brix” next to cell C1, “SP” next to cell3, and “%ABV” next to cell B7.

The spreadsheet makes the calculations very straightforward and easy.
 

sour_grapes

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However, if one measures the Brix number AND the specific gravity after fermentation, the %ABV can be calculated from the following formula:
Just to be clear: In the sentence above, when you say "Brix number," are you referring to the reading of a refractometer that is calibrated in Brix, but that is measuring a wine that has already gone through fermentation?
 

raguido

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Correct, the Brix number is obtained from the refractometer reading. I find that the refractometer reading is the easiest way to get the initial specific gravity. I've checked it against the measure SG and found the agreement to be very good. However, once fermentation starts and alcohol is produced, the refractometer reading is no long accurate for calculation of the specific gravity.
 

balatonwine

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I am always facinated by those so interested in estimating ABV, especially to a perceived decimal precision. Because, IHMO, it is one of the least important numbers as to the real end quality of many wines. Sure, it is a factor. Yes, it is required by labeling laws. But ABV estimation is just a motion to go through for me.

And it assumes a lot (precision of the measurement tools, amount of blow-off, amount of sugar made into alcohol versus yeast mass, real consistency of the must, etc, etc, etc..) Thus always a questionable number to estimate with before and after measurements alone. So much so, that even labeling laws allow a 1.0% to 1.5% error in actual versus declared ABV. Which is a lot, actually.

Sure, it is all fun to calculate (guestimate) ABV with a few cheap tools. But it still really is somewhat an amorphous, ballpark number only. If want to get a better handle on ABV, I send a sample to a lab.

Side note: If I forgot to take a brix/SG measurement at start (which I would find odd of anyone who is committed enough to have spent the money for a refractometer to forget to do), I would just shrug it off. What is the ABV? If asked, tell the truth. "Not sure. I did not take the correct measurements. But a nice little wine overall, isn't it?".
 

raguido

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When I am making five different batches of wine at the same time, it is easy to forget to take a refractometer reading. I just happened to miss one batch that time. Sorry that I can be as meticulous as balatonwine! I like to log all the characteristics to an Excel spreadsheet when making wine so that when I repeat a batch later, I'll know the exact conditions that were used the first time.
 

balatonwine

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When I am making five different batches of wine at the same time, it is easy to forget to take a refractometer reading. I just happened to miss one batch that time. Sorry that I can be as meticulous as balatonwine!
Naw. No problem. I said I found it "odd", that is all. Which only means my world perspective as I see it, but not necessarily complete, accurate or realistic in all circumstances. And I am of course very biased -- I grow. So of course I routinely measure brix to know when to harvest. Rather simply to point out, if I did forget or did not measure, oh well. That number really is not that important. The wine should stand on its own in the glass as what it is. Being too concerned about ABV by using complex estimates may miss that. As my opinion. No more. No less. So I trust no harm, no foul. ;)
 
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For my one gallon batches I hardly ever take a starting sg reading. Too hard to get my wine out of the bottle. I guess I should pour it and take a reading. If I am going close to a recipe on line, I will use what their ABV is.

Flavor is the most important but when I give a bottle out they always ask me what the alcohol content is. I always give an estimate unless I lie and say it is jet fuel. Lol
 

BernardSmith

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For my one gallon batches I hardly ever take a starting sg reading. Too hard to get my wine out of the bottle. I guess I should pour it and take a reading. If I am going close to a recipe on line, I will use what their ABV is.

Flavor is the most important but when I give a bottle out they always ask me what the alcohol content is. I always give an estimate unless I lie and say it is jet fuel. Lol
A plastic or stainless steel baster (you can pick one up at your local supermarket) will fit easily into the mouth of a gallon carboy (assuming it takes a # 6 (I think it is) bung. I use one to fill the cylinder that comes with the hydrometer. You can also get a more dedicated device called a wine thief but the truth is I have never found a way to get enough wine into it to be able to use that in place of the cylinder.

The thing is taking a reading of the gravity is often very useful - it can tell you when to rack from the primary, and it can keep you informed about how well (or not) the fermentation is going.
 

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