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Old 07-10-2012, 04:01 AM   #1
Mathblasta
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Default Determining ABV?

Hello, first post!

I recently made my first batch of brew not from a kit. It has made me wonder - How exactly does one determine approximate ABV from a homebrew? I took a starting gravity reading from my wort, but I don't know how to translate it into actual ABV. Is there an equation, or are the two not related enough to be relevant? If not, then how DO I get ABV?

Thanks in advance for any tips/advice!

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Old 07-10-2012, 04:10 AM   #2
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Og - fg * 131 = abv

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Old 07-10-2012, 04:14 AM   #3
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(Og-Fg)*131 is what I usually see. But as far as I know, that's an approximation that doesn't scale too well for higher gravity beers.

If you have starting gravity, but not final gravity, not much you can do.

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Old 07-10-2012, 04:15 AM   #4
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Originally Posted by UTPhish View Post
Og - fg * 131 = abv
Cool. Any idea what the reasoning behind that equation is? Like, why 131?

Also, reeeeally stupid question, but my fg is taken before I start bottle conditioning, or after I've got a ready to go beer?
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Old 07-10-2012, 04:16 AM   #5
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ABV calculators. You will need OG and FG. I Googled and found a couple. They seem to give slightly different values so when I use them I do several and average them.

Here is one:http://www.rooftopbrew.net/abv_calculator.php

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Old 07-10-2012, 04:17 AM   #6
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Welcome, Math. To determine your abv, you compare your original gravity to your final gravity - the gravity before fermentation and after fermentation. This tells you how much wort sugar was fermented, with alcohol being one of the byproducts.

I use brewing software, plug in starting and finishing gravity and if tells you the abv. You could also calculate it by hand, but I don't know the formula.

Edit: what they said ^^^^

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Old 07-10-2012, 04:21 AM   #7
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Quote:
Originally Posted by Mathblasta View Post
Cool. Any idea what the reasoning behind that equation is? Like, why 131?
Like most constants in most equations, I just chalk it up to an amount figured out by someone who had a lot more patience to do a lot more research than I care to do.

Quote:
Also, reeeeally stupid question, but my fg is taken before I start bottle conditioning, or after I've got a ready to go beer?
Before you start bottle conditioning.
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Old 07-10-2012, 04:23 AM   #8
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Quote:
Originally Posted by Mathblasta View Post
Hello, first post!

I recently made my first batch of brew not from a kit. It has made me wonder - How exactly does one determine approximate ABV from a homebrew? I took a starting gravity reading from my wort, but I don't know how to translate it into actual ABV. Is there an equation, or are the two not related enough to be relevant? If not, then how DO I get ABV?

Thanks in advance for any tips/advice!
If you assume attenuation is about 76%, which is generally a very good guess, then your ABV will be the same the OG (sorta). I.e., if the OG was 1.055 then ABV will be 5.5%. Works out exact if you just use gravity points (i.e., OG = 55)

If anyone wonders why, use the following two equations to solve for ABV (i.e., substitute or algabraic subtraction):
ABV = (OG-FG)*131
FG = (1-.76)*OG
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Old 07-10-2012, 04:24 AM   #9
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Quote:
Originally Posted by Qhrumphf View Post
Like most constants in most equations, I just chalk it up to an amount figured out by someone who had a lot more patience to do a lot more research than I care to do.

Also, reeeeally stupid question, but my fg is taken before I start bottle conditioning, or after I've got a ready to go beer?
Before you start bottle conditioning.[/QUOTE]

lol good enough reasoning for me!

As far as grav testing during the fermentation, how do I avoid oxygen/other stuff contamination?
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Old 07-10-2012, 04:27 AM   #10
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Quote:
Originally Posted by passedpawn View Post
If you assume attenuation is about 76%, which is generally a very good guess, then your ABV will be the same the OG (sorta). I.e., if the OG was 1.055 then ABV will be 5.5%. Works out exact if you just use gravity points (i.e., OG = 55)

If anyone wonders why, use the following two equations to solve for ABV (i.e., substitute or algabraic subtraction):
ABV = (OG-FG)*131
FG = (1-.76)*OG
Very cool, I had no idea.
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