Alcohol content

ckwatson · Mar 21, 2011

I have some questions about figuring out my alcohol content. When I use my specific gravity and do the math (OG-FGx105) gives me a ABV of 3.57 when I use the alcohol scale from my hygrometer it gives me a reading of 4.5 (6.5%-2%) Is this normal?

bob1852 · Mar 21, 2011

The formula used by most is:

OG-FG x 131...

Arneba28 · Mar 21, 2011

(OG-FG)x 131. thats what I was always told?

Yuri_Rage · Mar 21, 2011

The alcohol potential scale on your hyDrometer is probably less precise than the specific gravity scale, so using a formula to calculate ABV from specific gravity is likely the best way to get good results.

The formula you're using is for approximating alcohol by weight (ABW). The ones posted above are correct for alcohol by volume (ABV).

ckwatson · Mar 21, 2011

Thanks for the heads up on the ABW, when I use 131 I get very close to the 4.5% from the alcohol potential scale.

Captain Damage · Mar 21, 2011

The potential alcohol scale on your hydrometer is intended for winemakers. It doesn't work for beer with it's proteins and unfermentables.

beergynt · Mar 27, 2011

I think that OG-FGx105 was stated in Papazian's book... I tried it and soon found it to be wrong. I wound up playing with the numbers and comparing them to Beersmith's figures until I settled on 130, but I'm sure 131 is right there...

Calder · Mar 27, 2011

Both are correct.

x 105 will give you the percentage of alcohol by weight in the beer.

x 131 will give you the percentage of alcohol by volume in the beer.

unionrdr · Mar 27, 2011

I've been using the formula in the Cooper's directions. OG-FG/7.46 + .5= approximate %ABV.

Calder · Mar 27, 2011

unionrdr said:
I've been using the formula in the Cooper's directions. OG-FG/7.46 + .5= approximate %ABV.

Time to change. Check out a few web sites for calculators and you will see which is correct.

remilard · Mar 27, 2011

Calder said:
Time to change. Check out a few web sites for calculators and you will see which is correct.

Change in gravity x 131 isn't correct either. It is a linear approximation.

In order to accurately calculate alcohol you need to measure it directly using lab methods or know the real terminal extract, which also requires lab methods.

tchuklobrau · Mar 27, 2011

So then is gravity changex131 and accurate way to estimate abv? cause i was told it was OG-FGx.129 and my buddy was told yet another formula

bob1852 · Mar 27, 2011

It's kinda like converting brix to gravity.

99.9% of people will say brix x 4 = gravity, but the actual formula is:

{Plato/(258.6-([Plato/258.2]*227.1)}+1 = Specific gravity

But for the general homebrewing community OG-FG x 131 is acceptable...

Yooper · Mar 27, 2011

AZ_IPA said:
It's kinda like converting brix to gravity.

99.9% of people will say brix x 4 = gravity, but the actual formula is:

{Plato/(258.6-([Plato/258.2]*227.1)}+1 = Specific gravity

But for the general homebrewing community OG-FG x 131 is acceptable...

Yes, I agree. It's not 100% accurate, but for most of our purposes, (OG-FG)X 131= ABV is a darn good approximate.

unionrdr · Mar 28, 2011

Yooper said:
Yes, I agree. It's not 100% accurate, but for most of our purposes, (OG-FG)X 131= ABV is a darn good approximate.

So,why exactly is OG-FGx 131 more accurate than OG-FG/7.46 + .5? The first one gives a lot lower estimation than the 2nd. For example,my #'s plugged into both;1.044-1.012=.032x131=4.192%ABV...1044-1012=32/7.46=4.2895 +.5=4.7895.? In the 2nd one,you have to take out the decimal points to get the right calculation. In the 1st one,the higher multiplier tries to compensate for this. The 2nd one adds .5 for the effects of the priming sugar. So,the 1st one gives me 4.2%ABV,the second gives 4.8%ABV. The higher number "feels" closer to the truth. How do y'all really know which is correct?

Yooper · Mar 28, 2011

unionrdr said:
So,why exactly is OG-FGx 131 more accurate than OG-FG/7.46 + .5? The first one gives a lot lower estimation than the 2nd. For example,my #'s plugged into both;1.044-1.012=.032x131=4.192%ABV...1044-1012=32/7.46=4.2895 +.5=4.7895.? In the 2nd one,you have to take out the decimal points to get the right calculation. In the 1st one,the higher multiplier tries to compensate for this. The 2nd one adds .5 for the effects of the priming sugar. So,the 1st one gives me 4.2%ABV,the second gives 4.8%ABV. The higher number "feels closer to the truth. How do y'all really know which is correct?

I don't use priming sugar, for one thing!

unionrdr · Mar 28, 2011

Yooper said:
I don't use priming sugar, for one thing!

You don't prime your beers before bottling? Or do you keg,& it's not needed?

Yooper · Mar 28, 2011

unionrdr said:
You don't prime your beers before bottling? Or do you keg,& it's not needed?

I force carb in the keg, and if I bottle I bottle from the keg.

unionrdr · Mar 28, 2011

Ah,I see. I'd just really like a scientifically correct,yet simple answer to why the 1st one I listed is said to better than the Cooper's one? It gives a low reading. A lot lower than the 2nd one I listed. I think it may be because everyone uses it,& it's in a book,it's just got to be better.?...

Calder · Mar 28, 2011

Your are correct, we didn't account for the additional gravity from the sugar, but that would be less than 0.5% abv. Depending on how much priming sugar you add, you will add somewhere between .002 to .003 to the gravity (equivalent to .25 to .4%abv), but....... you are also adding additional water which will dilute that increase some. ........ Correct; it does slightly increase the final alcohol.

Where does the 131 come from?

Alcohol by weight = gravity difference x 105

Alcohol by volume = abw x 1.25

I'm sure both of those calculations have some rounding to them. The 131 comes from the product of 105 X 1.25.

For most of us, multiplying by 131 is simple and easy to remember, and is close enough.

wolverinebrewer · Mar 28, 2011

ABV=76.08*(OG-FG)/(1.775-OG)*(FG/0.794

From Real Beer.Com

Sometimes you can figure it out on your own, most often at a brewpub where starting and finishing gravity may be posted -- or at least you can ask about them. Here is a basic formula:

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).

You should remember that ABW is used mostly in the United States, while the rest of the beer world (as well as the wine and spirits world) measures Alcohol by Volume (ABV). That conversion is easy: ABV = ABW (FG/.794).

You can't do this unless the Original (Starting) Gravity is provided. You may calculate Final Gravity on your own if you have a hydrometer (available in any homebrew store). Just warm the beer to about 60F and pour it back and forth several times to cut down the carbonation. Then use the hydrometer to get a FG reading.

Not only will you be able to figure out the alcohol content, but this is a great conversation starter in brewpubs.

Figuring ABV by this method shows it a little higher than the difference in gravities * 131.

unionrdr · Mar 29, 2011

Calder said:
Your are correct, we didn't account for the additional gravity from the sugar, but that would be less than 0.5% abv. Depending on how much priming sugar you add, you will add somewhere between .002 to .003 to the gravity (equivalent to .25 to .4%abv), but....... you are also adding additional water which will dilute that increase some. ........ Correct; it does slightly increase the final alcohol.

Where does the 131 come from?

Alcohol by weight = gravity difference x 105

Alcohol by volume = abw x 1.25

I'm sure both of those calculations have some rounding to them. The 131 comes from the product of 105 X 1.25.

For most of us, multiplying by 131 is simple and easy to remember, and is close enough.

Well...maybe the .5 comes from using the Cooper's carb drops? They look/are the size of oval shaped throat lozenges. So no water is involved with these. Maybe it is a measure of maximum effect (.5)? Because they do carbonate beer really well when used in proper proportion.

Alcohol content

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