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Home Brew Forums > Home Brewing Beer > Brew Science > Determining % Alcohol
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Old 05-31-2011, 09:59 PM   #1
Tuckahoe
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Default Determining % Alcohol

By using my original SG and final SG readings, how can I determine the % alcohol of my brew?


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Old 05-31-2011, 10:02 PM   #2
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You multiply them together and then add 4.

Or you do a search.


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Old 05-31-2011, 10:06 PM   #3
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google search. ABV calculator. just type your readings and your temperature in there and it will tell you
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Old 06-01-2011, 03:15 AM   #4
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Originally Posted by Tuckahoe View Post
By using my original SG and final SG readings, how can I determine the % alcohol of my brew?
You can't, you can only do it with lab methods.

You can estimate it using that information with an approximation borrowed from wine making: 0.131 * (OG - SG).

You can estimate it more accurately using some formulae due to Balling which were summarized in an HBD post by George Fix several years ago (should be enough info to google it).

You can estimate it very quickly in your head as (OG -1) x 100%
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Old 06-01-2011, 04:37 AM   #5
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There are dozens of formulas out there. The aforementioned Balling formula multiplies the difference between the OG and FG (AE) by a factor which depends on the OG. For example if the OG were 12 P and the AE 3 (75% apparent attenuation) the ABW is approximately 0.421(12-3) = 3.789 i.e. 3.8%. Divide this by 0.791 for ABW. The multiplier doesn't change much in the region around 12 P (1.040) OG. Values calculated by this, or any of the other formulas, are approximations.
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Old 06-01-2011, 04:41 AM   #6
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You can estimate it very quickly in your head as (OG -1) x 100%
but that doesn't take the aggressiveness of the yeast into account at all. with that calc, windsor and wlp001 would be just as effective as nottingham.
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Old 06-01-2011, 04:43 AM   #7
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take the difference and divide by 7.69
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Old 06-01-2011, 04:45 AM   #8
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i just take the attenuation supplied by the average i find out on the internet, plug it into beersmith, and am extremely close ( or dead on )
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Old 06-01-2011, 07:10 AM   #9
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but that doesn't take the aggressiveness of the yeast into account at all. with that calc, windsor and wlp001 would be just as effective as nottingham.
Right, but all of the formulae you can plug OG and FG into and get a number don't take a number of things into account either. You can't tell what the alcohol content truly is without a lab. The question is to do you want to get out a spreadsheet and be close but wrong or do you want to do something in your head and be close but wrong?

I think you'll find that any two yeast will give fairly close attenuation in the same wort. The numbers reported by the yeast suppliers more reflects the fact that Belgian wort is more fermentable than English wort (for example) than it does that Belgian yeast is more attenuative than English yeast. Mash at 145 for 3 hours and add 3 lbs of sugar per 5 gallons and any yeast will get you 80%.
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Old 06-01-2011, 12:29 PM   #10
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but that [Ballings's Formula] doesn't take the aggressiveness of the yeast into account at all. with that calc, windsor and wlp001 would be just as effective as nottingham.
Yes, actually, it does, if by the agressiveness of yeast you mean their ability to ferment i.e variability in attained ADF. The Balling formula is

ABW = f*(OG - AE)

but AE = OG*(1-ADF) so

ABW = f*OG*ADF

ADF is expressed as a fraction here rather than a percentage.

I was too lazy to look up the formula for f last night but it is

f = 0.39661 + 0.0017091*OG + 1.0788E-5*OG*OG

with the OG in P. This is from a curve fit to the Balling Table as published in DeClerck, VolII p 428.

What Ballings formula does not take into account is that different yeast strains produce different amounts of biomass in the course of a fermentation. As the nominal production is 5.5% of extract consumed, and the tables are based on that, it is clear that doubling or halving of the yeast mass relative to the nominal doesn't have that much of an effect. It also relies on Tabarie's principle which, in my brief experiments with it seems a bit iffy. But then as you don't really know what your OG was (evaporation loss in the fermenter, dilution/concentration by the sugar content of the starter etc.) you couldn't really expect to be able to calculate alcohol content accurately from the formula anyway. It is, given this, often the practice that the alcohol content is measured and an effective OG calculated from that by using Balling's formula in reverse. Of course the limitations apply in the reverse direction as well so we seem to have a sort brewing Heisenberg principle: you can take a good OG measurement and calculate approximate alcohol content from that or take a good alcohol measurement and calculate approximate OG but you don't seem to be able to get both accurately.

There is another Balling formula

ABW = g*(OG - TE) where g is another factor depending in OG

g = 0.48394 + 0.0024688*OG + 1.5609E-5*OG*OG

and TE is the true extract. This takes Tabarie out of the picture and so is doubtless more accurate but the variability in biomass produced per unit of extract consumed is still problematical and few homebrewers are willing to go to the trouble to determine TE though it isn't really that difficult to do and requires minimal equipment in addition to a hydrometer i.e. a beaker and a volumetric flask.

The Balling equations may be the best out there because they take OG into account whereas the others don't but there are still limitations. In my experience the OG/AE form of the Balling equation, based on a specific gravity reading in the fermentor just after filling, underestimates the alcohol content of the beer by a few tenths of a percent. With respect to back calculation the effective OG is somewhat higher than the measured OG.


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