Membership questions? Log in issues? Email info@brewersassociation.org

Author Topic: Calculating ABV  (Read 4747 times)

Offline fyouberg

  • 1st Kit
  • *
  • Posts: 15
Calculating ABV
« on: October 07, 2013, 11:56:42 am »
Apologies up front if there is already a thread addressing this, but I can't find it if there is.  What formula can I use to calculate ABV in my home brews?  The Joy of Home Brewing provides the following formula:
(OSG-FSG)105
For purposes of comparison, lets let OSG = 1.080 and FSG = 1.020
This formula yields a ABV of 6.3%
Another formula I've run across is:
((OSG-FSG)1.05)/FSG/.79
This formula yields an ABV of 7.8%
That seems like a pretty large difference between two formulas that are supposed to be calculating the same thing.  Is there any consensus out there about a good formula to use (that does not require an advanced degree in mathematics)? ;)
If your yeast is happy, it can't help but be a good day.

Offline denny

  • Administrator
  • Retired with too much time on my hands
  • *****
  • Posts: 27070
  • Noti OR [1991.4, 287.6deg] AR
    • Dennybrew
Re: Calculating ABV
« Reply #1 on: October 07, 2013, 12:08:22 pm »
Drop the "1" from both OG and FG.  Then it's (OG-FG)*.132

(80-20)*.132 = 7.92
Life begins at 60.....1.060, that is!

www.dennybrew.com

The best, sharpest, funniest, weirdest and most knowledgable minds in home brewing contribute on the AHA forum. - Alewyfe

"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell

Offline dmtaylor

  • Official Poobah of No Life. (I Got Ban Hammered by Drew)
  • *********
  • Posts: 4716
  • Lord Idiot the Lazy
    • YEAST MASTER Perma-Living
Re: Calculating ABV
« Reply #2 on: October 07, 2013, 12:17:43 pm »
Drop the "1" from both OG and FG.  Then it's (OG-FG)*.132

(80-20)*.132 = 7.92

+1.  The formula from Papazian is for calculating alcohol by WEIGHT (ABW).  Denny's formula here is for ABV (Volume).  The two numbers are slightly different.  Homebrewers most often use ABV.  Commercial breweries I believe use ABW, or it depends on whatever the laws require at the moment as to how they label their bottles.
Dave

The world will become a much more pleasant place to live when each and every one of us realizes that we are all idiots.

Offline Joe Sr.

  • Official Poobah of No Life. (I Got Ban Hammered by Drew)
  • *********
  • Posts: 4467
  • Chicago - NORTH SIDE
Re: Calculating ABV
« Reply #3 on: October 07, 2013, 01:00:39 pm »
There are any number of on-line calculators you can use also.

FWIW, the formula I have always used is OG-FG*131.25 which would give you 7.875 on Denny's example.

I have no idea where I originally got the formula, but it's close enough to Denny's for me.  Rounding error and all that.  I'm too lazy to go change my spreadsheet for a couple decimal points.
It's all in the reflexes. - Jack Burton

Offline dmtaylor

  • Official Poobah of No Life. (I Got Ban Hammered by Drew)
  • *********
  • Posts: 4716
  • Lord Idiot the Lazy
    • YEAST MASTER Perma-Living
Re: Calculating ABV
« Reply #4 on: October 07, 2013, 02:10:45 pm »
Scientifically, in truth, the 131 or 132 or 128 that I have sometimes seen is only good to two significant figures, as there are other variables that affect this "constant" which makes it not truly constant.  So to take this calculation down to an extra couple of decimal points is... pointless.
Dave

The world will become a much more pleasant place to live when each and every one of us realizes that we are all idiots.

Offline Joe Sr.

  • Official Poobah of No Life. (I Got Ban Hammered by Drew)
  • *********
  • Posts: 4467
  • Chicago - NORTH SIDE
Re: Calculating ABV
« Reply #5 on: October 07, 2013, 03:09:01 pm »
I totally agree.  Even assuming our measurements were truly accurate, there is no real difference between 7.87 and 7.92.  As far as I'm concerned I would probably tell people that beer is around 8%.  All I really care about is a ballpark measurement, and I don't know that I care all that much.
It's all in the reflexes. - Jack Burton

Offline dordway29

  • Cellarman
  • **
  • Posts: 62
Re: Calculating ABV
« Reply #6 on: October 07, 2013, 04:29:43 pm »
you can multiply ABW by 1.25 to get ABV

(1.080-1.020)105*1.25 = 7.875

Offline HoosierBrew

  • I must live here
  • **********
  • Posts: 13031
  • Indianapolis,IN
Re: Calculating ABV
« Reply #7 on: October 07, 2013, 04:42:29 pm »
I totally agree.  Even assuming our measurements were truly accurate, there is no real difference between 7.87 and 7.92.  As far as I'm concerned I would probably tell people that beer is around 8%.  All I really care about is a ballpark measurement, and I don't know that I care all that much.
+1.  I go to the 1st decimal place when people ask how strong my beers are, but I know good and well that there is alot of variability there.
Jon H.

Offline fyouberg

  • 1st Kit
  • *
  • Posts: 15
Re: Calculating ABV
« Reply #8 on: October 07, 2013, 04:57:35 pm »
You are absolutely right.  I need to read a little bit more closely!  Having used Charlie's formula for ABW to calculate (incorrectly) my ABV, helps explain some of my friends comments like "Are you sure that this stuff is only x%!!"  Oh well, exceeding expectations is not a bad thing.  Thank you all once again.  What a cool community!

Drop the "1" from both OG and FG.  Then it's (OG-FG)*.132

(80-20)*.132 = 7.92

+1.  The formula from Papazian is for calculating alcohol by WEIGHT (ABW).  Denny's formula here is for ABV (Volume).  The two numbers are slightly different.  Homebrewers most often use ABV.  Commercial breweries I believe use ABW, or it depends on whatever the laws require at the moment as to how they label their bottles.
If your yeast is happy, it can't help but be a good day.