Calculating ABV

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Hi, I'm a little confused on how to calculate ABV. I'm using the Brewfather app and it saying the final ABV of my beer is 5.6%. I've also put the same starting and ending gravity readings into two online calculators and they say 5.51%. Why are they different?
 

Broken Crow

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Forgive me if this is naive, (I don't use software) doesn't (FG – OG) X 131.25 = ABV% work for you?
 

CascadesBrewer

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There was a good article in Zymurgy in the last couple years looking at ABV formulas. (ZYMURGY: JULY/AUGUST 2019 / How Much Alcohol is In Your Homebrew?) Looking at the article, the factor between formulas varies from 131 and 135. Based on analysis comparing measured ABV to calculated ABV, the author concludes that the factor 132.6 came very close to the measured ABV.
 

bracconiere

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(sorry just saw this was in begginers forum....)

but here it is simply....

10 mL × 0.790 g/cm^3 = 7.9 g of ethanol

density of sugar is 1.37178


but yeah i'd call the difference between 5.51% and 5.6% nominal....

just like i use the trick of multiplying or dividing by 0.8 ABV to get ABW....not spot on but close enough....

and i need to know how many grams i'm getting not Volume....
 
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Toxxyc

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Keep in mind that anything under 0.5% ABV isn't even strong enough to get you drunk, no matter how much you drink of it, so worrying about a difference between 5.51% ABV and 5.6% ABV on a homebrew level, for me, is just a waste of time. I typically work on the rounded value, so in that case I'd probably just say "5.6% ABV" to keep it easy.
 
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OG divided by 10 is pretty close to ABV (assuming 75% attenuation) for normal strength beers.

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eta: i used 131.
 
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Great question! In brewing, there are a couple of methods for calculating Alcohol % by Volume. These methods use different formula, numbers and science. Beer Calculators use a method that calculate ABV using PHYSIOLOGY, while Brewmeter uses a method that calculates using CHEMISTRY. Since ABV is a ratio, if we start with 2 numbers, we can find the 3rd. For example, if we start with 2 numbers 5 and 3, then we can determine the 3rd number is 150. Physiology method uses the rules of human body to find out alcohol content. First of all, it assume the weight of your body is average weight of 70kg. In other words, it assumes your weight is 70kg if you started with a weight of 70kg. It assumes the density of your body is 0.8g/ml. Therefore, it assumes your density is 0.8g/ml if you started with a density of 0.8g/ml. It assumes the volume of your body is 80L. In other words, it assumes your volume is 80L if you started with a volume of 80L. Therefore, for your body, Alcohol % by Volume = alcohol content x 0.8g/ml x 80L/70kg = (1 x 0.8g/ml x 80L/70kg) / (0.8g/ml x 80L/70kg) = 0.8. Therefore, final alcohol content by volume.
 

patto1ro

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This is what I use:

=((((OG-FG)*(125*1.05))+((OG-FG)*(1000/7.5)))/2)/1000

Can't for the life of me remember where I got it from. Some brewing text book or other.
 
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[1] =((((OG-FG)*(125*1.05))+((OG-FG)*(1000/7.5)))/2)/1000
  • 125*1.05 = 131.25
  • 1000/7.5 = 133.33

  • ((OG-FG)*(125*1.05)) ==> (OG-FG) * 131.25
  • ((OG-FG)*(1000/7.5)) ==> (OG-FG) * 133.33
  • Average the different estimates, divide by 1000
[2] = (OG-FG) * 0.1323 should give the same result to about 2 decimal places.

1655208837177.png
 

ScrewyBrewer

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There are two widely used formulas today for calculating FG, as far as I know. A short easy-to-do one by Charlie Papazian and a complex one by Ray Daniels claimed to be more accurate for high gravity beers. The results of which affect either of the ABV calculations.
 

patto1ro

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[1] =((((OG-FG)*(125*1.05))+((OG-FG)*(1000/7.5)))/2)/1000
  • 125*1.05 = 131.25
  • 1000/7.5 = 133.33

  • ((OG-FG)*(125*1.05)) ==> (OG-FG) * 131.25
  • ((OG-FG)*(1000/7.5)) ==> (OG-FG) * 133.33
  • Average the different estimates, divide by 1000
[2] = (OG-FG) * 0.1323 should give the same result to about 2 decimal places.

View attachment 771759
When you put it like that, it doesn't make a significant difference which of the two formulae you use. As I have it programmed into my spreadsheets, it doesn't involve any extra effort using the more complex formula.
 
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One would thing so, but periodically people ask why two different brewing apps calculate different values brewing estimation tools provide different results.
 

CascadesBrewer

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I just put my og and fg into beersmith and, hey presto, it tells me the ABV.

Yeah, this is what I generally do as well. The OG and FG readings I get with my hydrometer are only so accurate to start with. It is good to know if a beer is 4% vs 7% vs 12%, but I am not going to worry down to exact tenths.

The Brewer's Friend calculator (Alcohol By Volume ABV Calculator | Brewer's Friend) has a link to this article discussing the Standard and Alternate formulas you can pick. Sometimes I will use that to calculate a gravity for a high ABV beer since the Alternate formula is said to be more accurate for those.
 
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