Determining ABV?

Mathblasta · Jul 10, 2012

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!

UTPhish · Jul 10, 2012

Og - fg * 131 = abv

Qhrumphf · Jul 10, 2012

(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.

Mathblasta · Jul 10, 2012

UTPhish said:
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?

kh54s10 · Jul 10, 2012

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

Pappers_ · Jul 10, 2012

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 ^^^^

Qhrumphf · Jul 10, 2012

Mathblasta said:
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.

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.

passedpawn · Jul 10, 2012

Mathblasta said:
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

Mathblasta · Jul 10, 2012

Qhrumphf said:
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?

Pappers_ · Jul 10, 2012

passedpawn said:
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.

UTPhish · Jul 10, 2012

passedpawn said:
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

I always forget to correct my order of operations. (OG -FG) * 131 = ABV is correct.

I am not sure where the equation originated, but I am sure there are some math geeks on this forum that can explain.

meck1842 · Oct 10, 2012

I have also heard of, and use (go-fg) x 131.25. A couple of the online calculators come up with the same abv as this. Any explanation or has anyone else heard of this? I know its a slight difference but it is a difference none the less...

meck1842 · Oct 10, 2012

(OG*-fg)...

Qhrumphf · Oct 10, 2012

ABV =(76.08*(og-fg)/(1.775-og))*(fg / 0.794)

I've seen this one used, rumored to be more accurate at higher gravity.

ShinyBuddha · Oct 11, 2012

Qhrumphf said:
ABV =(76.08*(og-fg)/(1.775-og))*(fg / 0.794)

I've seen this one used, rumored to be more accurate at higher gravity.

Higher gravity being......? Anything above 1.060/1.080/1.100? Thanks for the equation though!

Qhrumphf · Oct 11, 2012

ShinyBuddha said:
Higher gravity being......? Anything above 1.060/1.080/1.100? Thanks for the equation though!

There's small variance across the board but in my observation it gets more pronounced when ABV exceeds maybe 7%. Above 10% it's pretty substantial. I haven't really paid attention to a gravity correlation. I'll play with some spreadsheets tomorrow running both side by side and see what happens.

freisste · Oct 11, 2012

I just had an explanation of the numbers, but I can't see my notes anywhere. Likely if you googled the phrase how to calculate ABV from gravity, it wouldn't take too long to find a good mathematical explanation.

I do know that the 131.25 takes into account multiple units, including gallons.

Qhrumphf · Oct 11, 2012

ShinyBuddha said:
Higher gravity being......? Anything above 1.060/1.080/1.100? Thanks for the equation though!

Just ran 34 different ones, covering 1.034 to 1.106, with a whole range in between, and varying FGs (with varying levels of attenuation). With the exception of one where the calculated ABV was actually lower under the more complex equation (4.2% down to 4.1%), in all cases the longer formula is equal to or higher than the simple one. Up until an OG of around 1.055-1.060, the increase is small, usually bumping ABV between 0 and 0.1 (say, 5.3% goes up to 5.4%). Above that, the factor slowly increases, and the change gets more noticeable. At an OG of 1.106, and an ABV of 11% with the simple formula, the longer formula gives 12.2%, which is a noticeable jump.

freisste · Oct 11, 2012

Ok, I've been doing a little research. Not sure how accurate you want to be. If ballpark is good, do what you have read above (with the 131.25 number).

If you are anal like me and plan on making an excel-based calculator (like me), read on...

To determine ABV, subtract FG from OG and multiply by a correction factor. This correction factor is based on the change in gravity and is not a constant. It ranges from 125 to 135. (I'm guessing 131.25 is a common, middle-of-the-road value that works pretty well.)

The site I found suggests using the following formula. They suggest that it is "probably the lost accurate formula you will find on any homebrew site". Based on this wording, I am assuming it is not perfect (note that their correction factor is based on empirical test data and is a line of best fit interpolated between test points).

ABV=(OG-FG)*(100.3*(OG-FG)+125.65)

This is what I will be using in the future. Note that for my first brew (OG 1.059, FG 1.010), the simplified formula says 6.6% and the above formula says 6.4%.

freisste · Oct 11, 2012

freisste said:
Ok, I've been doing a little research. Not sure how accurate you want to be. If ballpark is good, do what you have read above (with the 131.25 number).

If you are anal like me and plan on making an excel-based calculator (like me), read on...

To determine ABV, subtract FG from OG and multiply by a correction factor. This correction factor is based on the change in gravity and is not a constant. It ranges from 125 to 135. (I'm guessing 131.25 is a common, middle-of-the-road value that works pretty well.)

The site I found suggests using the following formula. They suggest that it is "probably the lost accurate formula you will find on any homebrew site". Based on this wording, I am assuming it is not perfect (note that their correction factor is based on empirical test data and is a line of best fit interpolated between test points).

ABV=(OG-FG)*(100.3*(OG-FG)+125.65)

This is what I will be using in the future. Note that for my first brew (OG 1.059, FG 1.010), the simplified formula says 6.6% and the above formula says 6.4%.

http://pint.com.au/calculators/alcohol/

Determining ABV?

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