Calculating final gravity

[wwright]

Just brewed an English brown ale that didn't boil off as much water as I was expecting. Was aiming for a 1 gallon batch size, starting from 2 gallons, but came out with more like 1.6 gallons. Therefore, I ended up with a relatively weak OG of 1.032. It will come out pretty watery and I'm coming to peace with that. My question is how would one calculate final gravity using math. I used Windsor dry ale yeast, which I think has a 75% attenuation rate, and 1.5 pounds of light malt extract. Sorry if this post is in the wrong area.

 

[Spectre216]

I believe your FG would just be 32 * .25, right?

 

[wwright]

Thats's what I was thinking, but remember it could be driven below 1 also. The .032 isn't the only fermentable sugar, because some beers can ferment down below 1 correct? But assuming the .032 was the only sugar to be fermented, I guess it would be 32x.25 to find what is left, correct?

 

[Complete_Amateur]

Thats's what I was thinking, but remember it could be driven below 1 also. The .032 isn't the only fermentable sugar, because some beers can ferment down below 1 correct? But assuming the .032 was the only sugar to be fermented, I guess it would be 32x.25 to find what is left, correct?

It can reach a gravity below 1.0 because alcohol has a lower density than pure water (1.000). A gravity below 1.0 will always be achieved with full attenuation of a wort with no (or few) unfermentable ingredients. The gravity will return to 1.0 when the alcohol is removed.

However, I think an effective formula for calculating and expected final gravity (with optimal conditions for the yeast), is:

FG (expected) = (OG - 1) * (1- Attenuation%)

I think this is what wwright is using.

 

[IL1kebeer]

It can reach a gravity below 1.0 because alcohol has a lower density than pure water (1.000). A gravity below 1.0 will always be achieved with full attenuation of a wort with no (or few) unfermentable ingredients. The gravity will return to 1.0 when the alcohol is removed.



However, I think an effective formula for calculating and expected final gravity (with optimal conditions for the yeast), is:



FG (expected) = (OG - 1) * (1- Attenuation%)



I think this is what wwright is using.

+1

Also, that formula is a good estimate and I use it as my max starting point for a new recipe; however, yeast can attenuate well above their manufacture's max range. Of course this is recipe and process dependent and not exactly typical, but I thought I'd share it for the OP to be aware of if he/she isn't already.


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