Well, this is going to be a bit complicated to explain so just let me take you to some sources of my info.
This is where I got the above formula:
http://www.realbeer.com/library/beerbreak/archives/beerbreak0301.php
"Alcohol percentage by weight equals 76.08 times Original Gravity minus Final Gravity divided by 1.775 minus Original Gravity. It is easier to scribble this down: ABW = 76.08(OG-FG)/(1.775-OG)."
Notice that I gave the formula for ABW not ABV by mistake. That's what I get for posting while distracted at work.
What I didn't post is that to convert ABW to ABV, The formula is this:
ABV = ABW (FG/.794).
Now look here:
http://beeradvocate.com/news/stories_read/518/
"Calculating the ABV
Say our brewer crafted a high-alcohol beer. The OG measured at 1.080, and the beer stopped fermentation with a FG measurement of 1.011. Simply subtract the FG from the OG and multiply by 131.
1.080 - 1.011 = 0.069 x 131 = 9.039%"
So we've got a 9 percent alcohol by volume beer. Easy!"
Now, let's test both ways:
OG 1.050
FG 1.010
Using the first formula from Real Beer for ABW, ABW = 76.08(OG-FG)/(1.775-OG)
76.08(1.050-1.010)/(1.775-1.050)
76.08(.04)/.725
3.0432/.725
ABW = 4.198
Now converting that to ABV, ABV = ABW (FG/.794).
4.198(1.050/.794)
4.198(1.322)
ABV = 5.550
Next, let's use the ABV formula from the other source, beeradvocate
(Beer: OG-FG(131)
1.050-1.010(131)
.04(131)
ABV = 5.24
Mmmmm which is it?
Let's try caluclations from here:
http://byo.com/feature/1315.html
"Final Gravity and Alcohol Content
The final gravity of a beer (in gravity points) can be calculated as:
FG = OG (1 - AA)
where OG is original gravity in gravity points and AA is the apparent attenuation as a decimal. Typical ranges of apparent attenuation of different yeast strains are given in the websites of yeast suppliers such as Wyeast and White Labs. (A value of 0.75 for AA gives a decent estimate for most yeast strains.)
Alcohol (ABV, alcohol by volume) can be calculated as:
ABV = (OG - FG) * 0.129
where OG and FG are the original and final gravity of the beer (in GP).
So using these formulas: OG-FG(.129)
1050-1010(.129)
40(.129)
ABV = 5.16
Damn Let's try another, You say
Charlie Papaizan says it's:
(FSG-OG)105=ABW
ABW.1.25=ABV
(1.050-1.010)105
(.04)105
ABW = 4.2
4.2(1.25) = ABV
ABV = 5.25
Now given all this data. We've tried 4 different formulas from documented sources to reach an ABV concensious. I'll be danged if there just isn't one.
So what is the ABV for our sample beer that started at 1.050 and ended up at 1.010?
5.25 (Papazian)
5.16 (BYO)
5.24 (beeradvocate)
5.55 (realbeer)
Take your pick. Do you want to brag about how much alcohol you generated? Use realbeer's formulas.
Do you want to convince your wife that you are not consuming as much alcohol as she thinks? Use BYO's formulas.
I really don't know which is the most accurate. I just know what I've learned from my sources and that it is not as simple as just subtracting OG from FG.