How do you find out what the specific gravity of a beer is supposed to be when it is finished fermenting?

Question

The starting specific gravity before pitching yeast was 1040 it has been 6 days and all signs of fermenting are gone. I am wondering what the specific gravity should read when completed. I am using an ironmaster irish stout extract kit,with an ounce of northern brewer pellet hops, and a can of dark malt to replace sugar. Please help! Am I ready to bottle? The hydrometer now reads 1020.

Answer 1

1.020 is a fairly high finishing gravity, and indicates a sweeter beer. You are ready to bottle once for 3 days, the gravity doesn't change (much).

Also, your alcohol can be calculated thus:
40-20 = 20
20 * 0.125 = 2.5%ABV.

This makes me think that either your initial hydrometer reading was off, the yeast is not done fermenting (though it will not ferment more if it stays the same a few days, leading to what we call a "stuck fermentation" ... google for remedies), or this is just a light sweeter beer.

Answer 2

General rule of thumb is that it's done when the final gravity is around 1.005-ish. It will all depend on your recipe and what final result you're after. 1.040 is a fairly light starting gravity, but if you're presently at 1.020, you're only slightly past halfway there. You may have a stuck fermentation. Next time you take a gravity reading, taste your sample and see if it's still sweet. That will be a good indicator beyond what your hydrometer tells you. You might consider racking it to secondary fermentation and let it sit for another week...6 days is still pretty early.