Author Topic: Formula for expected FG when OG is known?  (Read 1200 times)

Offline Lazy Ant Brewing

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Formula for expected FG when OG is known?
« on: February 25, 2017, 03:18:44 PM »
I like experimenting with my own brew recipes.  I can measure OG with a hydrometer and FG after fermentation is complete, but is there a formula that I can use to calculate an expected FG before I brew the recipe? 

What I normally have done is to find a similar published recipe with a similar OG and guess my FG might approximate that.

P.S. I do  my own calculations in excel.

Thanks in advance for your advice.
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Offline chezteth

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Re: Formula for expected FG when OG is known?
« Reply #1 on: February 25, 2017, 03:27:38 PM »
Beer software like beersmith calculates the FG. But, there are so many variables that it is only a guess. Yeast % attenuation, mash temp, grain bill, etc all effect the outcome. I don't know of any equation that would do what you are looking for.

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« Last Edit: February 25, 2017, 03:30:50 PM by chezteth »

Offline HoosierBrew

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Re: Formula for expected FG when OG is known?
« Reply #2 on: February 25, 2017, 03:34:10 PM »
I like experimenting with my own brew recipes.  I can measure OG with a hydrometer and FG after fermentation is complete, but is there a formula that I can use to calculate an expected FG before I brew the recipe? 

What I normally have done is to find a similar published recipe with a similar OG and guess my FG might approximate that.

P.S. I do  my own calculations in excel.

Thanks in advance for your advice.


No, not with any degree of accuracy. There are just too many variables like grist, mash temp, particular yeast strain, yeast health and quantity. And software estimates are never to be trusted - they're rough estimates at best. It's kind of a cliche here, but the yeast is done when it's done. You can learn over time what to expect for FG when you brew a recipe enough times.
 
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Offline BrewBama

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Formula for expected FG when OG is known?
« Reply #3 on: February 25, 2017, 03:37:17 PM »
You can estimate the approximate finishing gravity of a beer by taking into account the attenuation rate of the yeast strain you are using.  For example if you have a yeast with a 75% attenuation rate and your original gravity is 1.050 the estimated final gravity would be about 1.012. 

Like Chezteth and Hoozierbrew said, this is merely a estimate and can be lower or higher because there are many factors that affect the final gravity.


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Offline Andy Farke

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Re: Formula for expected FG when OG is known?
« Reply #4 on: February 25, 2017, 04:38:33 PM »
BrewBama has the basic equation, but you'll want to adjust for mash temperature if you're mashing really high or really low.

I've had quite decent luck with BeerSmith and its calculations. If I hit mash temps, I usually get a final gravity within a point on either side of the estimated value, which ain't bad. Looking through the settings, BeerSmith's default is to reduce attenuation by 1.25% for each degree above 153.5, and increase it by 1.25% for each degree below 153.5. This adjustment could be incorporated into your Excel formula to match the BeerSmith results (and you could further tweak it for your system as you collect results over time).

[for some reason I can't exactly match the BeerSmith values when I calculate a test equation out in Excel; I suspect it's something with the way the software calculates or assumes another aspect of attenuation? Or maybe a rounding issue? In any case, I'm still within 0.001 of the BeerSmith values, and that's pretty much in what I consider standard measurement error on my own refractometer and hydrometer]
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Offline stpug

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Re: Formula for expected FG when OG is known?
« Reply #5 on: February 25, 2017, 04:52:44 PM »
is there a formula that I can use to calculate an expected FG before I brew the recipe?

I do this for every beer I brew, and I take note of my expected FG on brewday.  I usually finish very close, but there are always exceptions.  It starts by knowing the yeast strain you're using and how it attenuates for you in your process.  This requires actually using it yourself and taking note of it's performance.

So, once you know how a certain strain of yeast performs for you then you can estimate your FG by simply using the non-attenuation percentage (e.g. 100% - 75%AA = 25%nonAA) against your OG_points (i.e. OG 1.065 - 1.0 = 65 OG_points).

Example 1) YeastX has 75%AA
OG_points x nonAA% = ~FG
1.065 x .25 = 16.25 (means FG 1.01625 @ 75%AA)

Example 2) WY1056 = ~80% AA
1.065 x .2 = 16.25 (means FG 1.013 @ 75%AA)
« Last Edit: February 25, 2017, 04:55:12 PM by stpug »

Offline denny

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Re: Formula for expected FG when OG is known?
« Reply #6 on: February 25, 2017, 05:15:52 PM »
Attenuation has a lot more to do with wort composition than yeast attenuation rating.  Depending on the wort, I can get from 60-85% from the same yeast.
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Offline stpug

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Re: Formula for expected FG when OG is known?
« Reply #7 on: February 25, 2017, 05:29:49 PM »
Attenuation has a lot more to do with wort composition than yeast attenuation rating.  Depending on the wort, I can get from 60-85% from the same yeast.

^^What Denny says is a valid point and the reason I specified:
"It starts by knowing the yeast strain you're using and how it attenuates for you in your process.  This requires actually using it yourself and taking note of it's performance."

Offline Andor

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Re: Formula for expected FG when OG is known?
« Reply #8 on: February 25, 2017, 05:47:07 PM »
Here's an article about attenuation. It does give the formula for attenuation, you could get a rough guess by solving for FG and using a average attenuation number- 70-75%. I'd say looking at past batches is a better approach though.

http://byo.com/malt/item/1895-attenuation-advanced-brewing



Offline HoosierBrew

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Re: Formula for expected FG when OG is known?
« Reply #9 on: February 25, 2017, 06:06:52 PM »
Attenuation has a lot more to do with wort composition than yeast attenuation rating.  Depending on the wort, I can get from 60-85% from the same yeast.


Totally agree, Denny.
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Offline Lazy Ant Brewing

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Re: Formula for expected FG when OG is known?
« Reply #10 on: February 25, 2017, 07:39:47 PM »
Thanks to all.  This gives me something to work toward.
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Offline brewinhard

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Re: Formula for expected FG when OG is known?
« Reply #11 on: February 25, 2017, 09:59:46 PM »
I like experimenting with my own brew recipes.  I can measure OG with a hydrometer and FG after fermentation is complete, but is there a formula that I can use to calculate an expected FG before I brew the recipe? 

What I normally have done is to find a similar published recipe with a similar OG and guess my FG might approximate that.

P.S. I do  my own calculations in excel.

Thanks in advance for your advice.


No, not with any degree of accuracy. There are just too many variables like grist, mash temp, particular yeast strain, yeast health and quantity. And software estimates are never to be trusted - they're rough estimates at best. It's kind of a cliche here, but the yeast is done when it's done. You can learn over time what to expect for FG when you brew a recipe enough times.

This, to the T.

Offline dmtaylor

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Re: Formula for expected FG when OG is known?
« Reply #12 on: February 25, 2017, 10:47:21 PM »
I like experimenting with my own brew recipes.  I can measure OG with a hydrometer and FG after fermentation is complete, but is there a formula that I can use to calculate an expected FG before I brew the recipe? 

What I normally have done is to find a similar published recipe with a similar OG and guess my FG might approximate that.

P.S. I do  my own calculations in excel.

Thanks in advance for your advice.

An average yeast under average conditions will give you approximately 75% apparent attenuation.  Do you agree?  Then do this:

FG=(OG-1)/4+1

In Excel, this looks kind of like:

        A
1     OG
2   1.060
3
4     FG
5   =(A2-1)/4+1

Hit enter in A5 and it should spit out the expected result: 1.015.

You can toy around with percentages and decimals instead of dividing by 4, but this will give you a swaggy ballpark result.

Others are of course correct when they say there are many variables, the primary one being yeast strain, followed by grist, mash time, mash temperature.  I keep close track of average attenuation for every yeast strain because I know it makes the biggest difference of all.
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Offline Mythguided Brewing

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Re: Formula for expected FG when OG is known?
« Reply #13 on: February 26, 2017, 09:15:22 AM »
I can't remember where I saw it (probably buried somewhere in Palmer's "How to Brew"), but the method I use in Excel to estimate FG from OG is:

=((OG Points/5) + (OG Points/4))/2

So let's say OG of 1.050, so OG Points = 50.

50/5 = 10
50/4 = 12.5
Average = 11.25

So estimated FG would be 1.011

Offline Richard

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Re: Formula for expected FG when OG is known?
« Reply #14 on: March 02, 2017, 11:59:26 PM »
Doing a wee bit of algebra on the equation:
((OG Points/5) + (OG Points/4))/2 = (0.2*OG Points + 0.25*OG Points)/2 = 0.225*OG Points

This is the same as assuming 77.5% attenuation, not very different from the generic 75% discussed above.