How do you determine "target" FG reading?

[Rob2010SS]

Hey guys. What determines what your FG reading should be after fermentation? Is there a calculation that determines what you should finish at? I guess here is what I'm trying to answer. My Grapefruit Pulpin OG reading was 1.064. Assuming it's ok to break open primary fermenter and take a reading, what SHOULD the final gravity reading be? How do you figure that out?

I know, noob question, sorry.

 

[lucashtorres]

Hello there,
I'm new here so there is people with more knowledge than I do. You should try using Beer Smith or Beer Friend, which are softwares you can add your recipe and they estimates your FG. You can also after the 7th fermentation day read your gravity and do so 3 days later. If its the same, you are probably done. If it has changed, wait a few more days and check it again.
Hope it helps!

 

[billl]

If you bought a kit, it should come with an estimate.

If not, each yeast has an approximate range listed. eg 70-80%. That means they produce finished beer where the gravity drops 70-80%. If you used that yeast, you would expect your beer to finish in the 1.012 -1.019 range (of course I have no idea what yeast you used) Even then, it approximation.

Whether you should open it up and take a gravity reading really depends on how long it's been fermenting. Don't even think about opening it up for at least a week.

 

[kh54s10]

You take the OG. Then look up the attenuation of the yeast you are using. If it attenuates to 73 - 77% your beer will have a target of 73 - 77% of your OG.

Here is a calculator: The second section: http://www.angelfire.com/wi3/johnsons/abv.html

 

[SlitheryDee]

I assume all the brewing softwares arrive at an estimated FG by an algorithm of some kind that takes into account the sugar composition of your fermentables, the relative percentage of the grist that those fermentables make up, and the kind of yeast you use to ferment.

That seems like a lot of work to do by hand, and honestly I doubt you really need to. Even if you did it, you'd only have a rough estimate. Just knowing generally where you're supposed to be is enough. I'd expect a beer with an OG of 1.064 to attenuate down to the 1.010-1.015 range depending on just how much of the grist was specialty malt or other less fermentable ingredients. If you get a reading anywhere close to that that is stable for 3 days you're done.

 

[Biscuits]

It's pretty easy, but keep in mind it always just an estimate...the attenuation numbers on the yeast package are not perfect. The easy math is to take the last numbers of the gravity reading, in your case 64, and multiply is by (1-percentage of attenuation in decimal form)...so:


64 * (1-.75) = Est. FG


So, assuming your est attenuation is 75%, your FG should be somewhere around 1.016.

 

[flars]

Attenuation ranges given by the yeast manufacturers are only good for comparing yeasts. The final gravity of your beer will depend upon the amount of fermentables sugars in the wort.

The Dead Ringer I have brewed many times has an OG of 1.064. With WY 1056 or SA-05 will consistently finish at 1.007 to 1.008.

A recently brewed porter held a steady SG of 1.014 for 6 days after 14 days in the primary. The yeast was WY 1332. A month later the porter was becoming over carbonated. SG of the bottled beer was 1.007.

Every situation is different. I would not trust a calculated FG and then go to bottling. Kegging is a little safer if the fermentation had not finished.

Not rushing to packaging your home brew will often result in a better beer to pour.

 

[Rob2010SS]

Wow, lots of good replies. Thanks guys, that helps a lot. I guess I just needed to know how to get there.

I am freaking pumped about this hobby. I know it's only my first batch but it's a lot of fun.

Thanks guys.

 

[kh54s10]

I would also suggest that you don't worry yourself about target attenuation. Use it as a guide. If the target says you should have 1.010 and you have 1.035 there is a problem. If you are close (within the range of the calculator I linked) or even outside a couple of points it is good. What is critical, if you are bottling, is that your final gravity is stable. Get the same gravity readings 2 days apart. If you bottle and there is more fermentation than just the priming sugar you could produce bottle bombs.