Local homebrew store owner today told me that the FG is roughly a certain % of the OG. Im curious, as Ive been making some of my own recipes now, and Im not sure what to look for as my target for FG. She said its about as good as it'll get, (OG 86, currently 20). Anyone have input on this?
When brewing with extract (rather than mashing all the grain yourself), typically the gravity will end somewhere around 1.020, due to unfermentable sugars in the extract. So your shop owner is probably correct. But as another person said, check the gravity, wait a couple of days, then check again. If the gravity hasn't changed at all, then you'll know for sure that fermentation is complete. Especially if your OG was 1.086, I'm sure you won't get much lower than 1.020. This brew will be high in alcohol! What kind did you make?
unfermentable
The intent was a fairly high alcohol content, the idea was to make an approximate clone of an imperial stout that was about 7.8%:
1.25 lb chocolate .25 lb roasted barley .5 lb crystal 8.8 lb liquid munich amber malt 2.7 lb wheat DME 8 oz malto-dextrin centennial (bitter) kent goldings (bitter) total hbu = 11.3 .5 oz cent, flavor .5 oz kent golding, flavor and some hop for finishing/dry hop
It is only VERY ROUGHLY a certain %... because different extracts have different amounts of fermentable vs. unfermentable sugars, and therefore, will ferment down a different amount. This difference can be considerable. Also, some yeast strains will ferment more aggressively than others. Another example is that Lager yeast can ferment an extra type of sugar that ale yeast cannot, therefore lager yeast should ferment a little further than ale yeast. Other additives, like maltodextrin and crystal malts, will not totally ferment out and, therefore, raise the FG higher than expected.
With all the variables, I don't think you can count on figuring the FG simply by using a % of the OG. The only exception might be if you always use the exact same ingredients and derive the % from experience.
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