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)? 
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.
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.
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.
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.
you can multiply ABW by 1.25 to get ABV
(1.080-1.020)105*1.25 = 7.875
+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.
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!