Calculating %ABV Without the Initial Specific Gravity

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raguido

Member
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.

bernardsmith

Well-Known Member
Hi Raguido - and welcome. Don't you need to know the initial gravity( or Brix) to be able to use a calculator to determine the actual ABV? I can start with any SG (or Brix) and ferment down to a gravity, say , of .996. How on earth will a calculator be able to tell me whether my starting gravity was 1.100 or 1.090 or 1.050 and if it cannot do that then the calculator cannot in fact tell me whether this batch is 13% or 12% or 7% ABV. My hydrometer (as does my refractometer) has a column for Brix and one for SG. (Rule of thumb Brix is about 1/4 of SG) These are simply alternative ways of measuring density. I guess I don't understand how any calculator can provide a datum about initial density that cannot be known from a final density.

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raguido

Member
Yes, you can measure Brix with a refractometer to calculate specific gravity, but only in the ABSENCE of alcohol, i.e., before fermentation starts. If you use a refractometer later when fermentation is well underway, there will be substantial alcohol present. The value of specific gravity calculated using Brix under those conditions will be inaccurate. (You can try that yourself to confirm that.) Normally, one can calculate the %ABV from the initial and final specific gravities, but if you forgot to measure the specific gravity prior to fermentation, you won't be able to do that, which is why the Brix at the end needs to be measured ALONG with the specific gravity to do that calculation. The formula that I presented allows you to do the calculation from these measured parameters.

bernardsmith

Well-Known Member
How? A practical example: My final gravity is 1.008 which is the same as a Brix of 2.05. What does your calculator tell me about the ABV of this wine? Unless your point about knowing the Brix is knowing the Brix of the fruit before I pitched the yeast but knowing the Brix of the fruit is exactly the same as knowing the SG at the start. If that is all your calculator does all it does is convert Brix to SG and as I said in my response to your first post - you can more or less assume that SG is the same as 4 times the Brix.

OP
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raguido

Member
You can't calculate the %ABV from just the final SG. You need to have the initial SG as well. In my case, I didn't have that number so I couldn't calculate it as is normally done:

%ABV = (SG_initial - SG_final)/0.0074

If you know the initial SG, then you can calculate the %ABV in this way. I didn't have that number for that batch of wine. But if I have BOTH the final SG as well as the Brix pf the wine measured with the refractometer, I can then calculate the %ABV via the formula listed in my first posting.

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bernardsmith

Well-Known Member
Simpler, if you know the starting gravity and the finishing gravity to multiply the difference by 131 (ball park accurate enough)

kh54s10

Supporting Member
HBT Supporter
I don't have time to test this so I'll take you word. But, it doesn't sound right. You are taking a refractometer reading and a hydrometer reading of the finished wine. Without knowing the original gravity or the original brix, how does the calculation take into account the attenuation achieved. For instance one yeast may take a wine from 1.100 to .998 and another may take the same wine from 1.100 to .990. The two would be different, but the difference between the final brix and final SG readings should be similar.

jgmillr1

Supporting Member
HBT Supporter
But if I have BOTH the final SG as well as the Brix pf the wine measured with the refractometer
Yes it is theoretically possible to measure both the hydrometer SG and a refractometer "brix" value to estimate the residual sugar and alcohol in a beverage. This is because the index of refraction (reported by the refractometer as brix) will respond the to both the dissolved sugar and the alcohol with a different relationship than the way the SG hydrometer measurements respond. However careful calibration must be made and the SG must be corrected for temperature. Also the relationships are not purely linear. So, it is theoretically possible but practically difficult.

Owly055

Well-Known Member
Here is a page describing the process......... I personally consider it a bit absurd. Accuracy is liable to be poor without extremely high quality devices, and a closely controlled and corrected procedure.

https://www.bellinghamandstanley.com/ltd/abv_kit.html

And it begs the question "what does it matter?" Final ABV is final ABV, and there is nothing you can do about it except fortify or dilute. The reason we take FG readings is to ensure fermentation is complete, the actual ABV result is just a matter of curiosity, not a value that is of any real use to us. Do I really care if my ABV is 5% or 6.5%? And if I know, what will I use that information for in the real world?
The only real reason I take an OG is to know how efficient my process is. The only reason I ever take an FG is to ensure fermentation is complete. I've often brewed taking neither reading, and you know what? The beer is indistinguishable from beer I took readings on. I do not bottle with crown caps. I mostly keg now, and when I do bottle, it's with EZCap bottles, rated at well in excess of 100 psi, that I know will vent long before they can grenade, though I've never had that problem. I often use a hydrometer to monitor conversion, particularly when doing a very short mash, as with last week's brew, when I did a 15 minute mash to increase body, as I lacked any carapils. Also with my home malted grain, I take refractometer readings during the mash due to the lower diastatic power. If I'm not satisfied, I will add some amylase, give it a bit more time, and recheck. It serves in lieu of an iodine test.

H.W.

bernardsmith

Well-Known Member
right, but beer ain't wine and wine ain't mead. A mead maker can have a starting gravity of 1.150 (Nuts I know, but it takes only 4.25 lbs of honey in that gallon to hit that SG and that SG will produce an ABV of about 20% and realizing that the mead is likely to be rocket fuel may encourage the mead maker to find ways to reduce that outcome but unless you know the starting gravity you have no way of being able to determine it by simply knowing the final gravity OR Brix. However, if you KNOW the starting gravity (or Brix) then you CAN use a refractomer to calculate/measure the gravity or Brix even if there is alcohol present but THAT is not what raguido is suggesting.

dyqik

Well-Known Member
There is an article in one of last year's Zymurgys with calculations for doing this for beer. This does work, and I've checked it in a couple of beers, a 4.2% bitter and an 8.2% dubbel (really as a way of checking my wort correction factors for the refractometer).

For wine, you need different correction factors, but it should work. What you are doing is using the refractive index of the finished wine measured by a refractometer and the specific gravity of the finished wine measured by a hydrometer to determine the amount of alcohol in the solution.

Two unknowns (concentration of residual sugar, concentration of alcohol) and two measurements (final brix and final gravity) means that you can solve the equations. But yes, accuracy is a consideration.