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Calculating ABV

I Brewed A Belgian Grand Cru and was told when I bought the ingrediants that it should finish at about 8-9 Abv my OG was 1.072 and FG was 1.020 The Calculations I used was 1.072-1.020=0.052x105=5.46ABV. Any Ideas why I finished on the light side.

Ingrediants were: 2 cans of wheat Malt extract, Specialty grains,1 lb.white wheat malt, 1 lb. weyermann cara foam, 2 lbs of honey w/15 min left in boil, & spices. 2 hop additions. THANKS.

I get 6.8?

OG - FG x 131 = ABV

1.072 - 1.020 x 131 = 6.8%

I saw the same equation you did at one point about a year ago and was way off on all my estimated ABV numbers. Don’t remember where but I did see OG-FGx105, but it’s really the equation above.

Sounds like the people who told you that looked at a hydrometer and read the potential alcohol off the OG and didn’t subtract for the FG.

Papazian says the following on page 43 of Complete Joy of Homebrewing:

(OG-FG) X 105 = ABV

homebrewzone.com says the following:

(OG-FG) X 129 = ABV

beeradvocate.com says the following:

(OG-FG) X 131 = ABV

I know you didn’t ask but I use these sites:

http://www.rooftopbrew.net/abv_calculator.php - ABV http://www.probrewer.com/resources/tools/beerspecs.php - Specs

Might as well put this one into the mix this is the one I use.

http://www.angelfire.com/wi3/johnsons/abv.html

I think there was only once I put the effort into taking a reading before AND after fermentation, but can’t you just take the original potential alc % and subtract the final potential alc %?

So you don’t take gravity readings? Ever?

[quote=“dobe12”]OG - FG x 131 = ABV

1.072 - 1.020 x 131 = 6.8%

I saw the same equation you did at one point about a year ago and was way off on all my estimated ABV numbers. Don’t remember where but I did see OG-FGx105, but it’s really the equation above.[/quote]

I think the equation with the 105 is to calculate ABW

[quote=“BPBCo”][quote=“dobe12”]OG - FG x 131 = ABV

1.072 - 1.020 x 131 = 6.8%

I saw the same equation you did at one point about a year ago and was way off on all my estimated ABV numbers. Don’t remember where but I did see OG-FGx105, but it’s really the equation above.[/quote]

I think the equation with the 105 is to calculate ABW[/quote]

Exactly^^…1.072-1.020 is 5.4 ABW or roughly 7.0 ABV.

Thats where I got the formula from Pappazian’s book. I’ll go with the 131 that brings me to almost 7% I’m OK with that!! Thanks to All!

Not really. I usually let the primary go at least 3 weeks and just figure the beer has done what it is going to do. Haven’t had any issues with stalled fermentation or anything yet. As far as telling how strong it is my calculations include how much I have eaten and how many beers I have had. I brewed a partigyle red that I did take readings on and have a breakfast stout coming up that I will hopefully remember to measure at kegging time.

Anybody have input on subtracting OG potential from FG potential?

If you mean the potential alcohol scale on a hydrometer, then that’s the same as 131(OG-FG).

Wow, didn’t know about multiply-by-131.

That puts my first homebrew (also a Grand Cru) in the 6.4% range.

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