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Old 10-20-2011, 04:43 PM   #1
Oct 2011
Orlando, FL
Posts: 33
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I had a few questions about specific gravity that I was hoping someone could help me wrap my head around.

I see a bunch of people saying that they hit their OG, or were off by a bit. Is that because you have a recipe that says what it should be, or are you figuring it out somehow based on something? I'm brewing a kit that doesn't tell me what my OG should have been (at least I didn't see it), so I don't have anything to reference; I was just wondering if there was something I could use to estimate what it should have been.

Secondly, OG versus FG: the kit I have also doesn't mention what FG I should be getting to. I'm assuming that I could just measure until it levels off and then I know fermentation is complete, but I was wondering what the relationship of OG versus FG was - is it just to figure the ABV, or are they nice numbers to know for repeatability of brewing?

This is more for my future knowledge-base as I currently don't have a hydrometer and I'm (ignorantly) following the kit's directions of letting it ferment for two weeks and then bottle. Next brew is going to be a SMaSH of some type and I'll make sure I'm set to check SG's.

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Old 10-20-2011, 04:51 PM   #2
stratslinger's Avatar
Dec 2010
Terryville, CT
Posts: 2,545
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Good questions, all of them!

On OG - most kits will tell you what you should expect for OG - whether those kits are extract, PM, or AG. However, it's also possible to plug in your recipe to one of several brewing software tools to calculate what your OG should be.

On FG - you've got the right idea. Even if a kit or recipe tells you what you should expect for FG, it's always best to wait until you've got a consistent gravity for at least 3 days running. I recently made a Wiezenbock that finished 7 points lower than the recipe said it should - had I bottled as soon as it hit the recipe's numbers, I probably would've wound up with a buncha bottle bombs!

As for how it relates to OG, that depends on a few factors. Fermentability of your wort is a big one - you don't have a ton of control over this in extract brewing, but all grain brewers definitely have to be aware of it. Secondly, your yeast strain plays a big role - some yeasts attenuate (convert sugars to alcohol) at a much higher rate than others.

While the OG and FG numbers are definitely key to figuring alcohol content, those numbers can be useful when brewing the same recipe multiple times. If the numbers are consistent, along with several other factors, then you can probably expect similar results from the brew. If they're not, well, you could be in for a surprise (some good, some not so much).

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Old 10-20-2011, 04:55 PM   #3
Hang Glider
Beer Drinker
Hang Glider's Avatar
Mar 2007
North Augusta, SC
Posts: 3,218
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Odd that your kit does not include these numbers!

Contact the vendor - ask!
Extract kits will mostly hit their Original Gravity numbers; if you don't get the same number it may be due to stratification. Very thorough stirring prior to adding the yeast may help.

Knowing the gravities is not essential, but how do you know when it's finished?
(time!) If you see foaminess (Krausen) after the first couple of days, it's doing its job. now leave it be for 3 weeks... prior to transferring, bottling, whatever.

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Old 10-20-2011, 05:18 PM   #4
nilo's Avatar
Aug 2009
Seattle, WA
Posts: 732
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Usually, recipes and kits would tell you the expected OG and FG. If it doesn't, you can plug the ingredients in a free online brewing tool (like beertools.com) and estimate both numbers, including the bitterness (IBU), color (SRM) and alcohol by volume (ABV%).
ABV% can easily be also estimated as (OG-FG)*129.
FG is related to OG as FG=OG-(OG-1)*attenuation, where the attenuation in % is the reduction of sugars that the yeast will provide. This amount is indicated by yeast strain from the manufactor, like WY1056 attenuation is from 73 to 77%.
So if your OG was 1.050, you could estimate your FG as 1.050-(1.050-1)*0.75=1.013, if using an average attenuation value.
Note that FG is also affected by the actual recipe, which extract was used, which grains, use of dextrin or lactcose, fermentation temps, mashing temps if doing all grain process.
Yes, definitely get a hydrometer to be in control of your fermentation.
If you are curious (and a freak like me) on how a yeast work the sugars, here's a graph of gravity taken daily for WY1007 and fermentis S04.
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