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Say I am brewing a beer that is supposed to have an OG of 1.050. I'm doing all-grain brewing and at the end of my mash & sparge, if I took the gravity at that point, what should I expect?

I think because I am pre-boil, the gravity at that point would be somewhere lower than the expected 1.050, but can I calculate that to make sure I am on the right track?

If I can figure out an expected post-mash gravity, if I do not hit it, are there adjustments I can make at that point? Boil longer? Add some extract?

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Yes, you should expect a lower pre-boil gravity due to dilution. The math is straightforward: gravity is a measure of the ratio of sugar to water; if you boil from, e.g. 6g to 3g, you still have the same amount of sugar, and thus the gravity changes by 6/3 or ×2. Similarly, if you want 5g at the end of a 60 minute strong boil, you should be collecting ~6.5g out of the mash, and the pre-boil gravity will be 5/6.5 = ×0.77

Another option is to do nothing. Unless the OG is significantly off, enough to push the beer out of style/category (if you even care about such things), RDWHAHB¹. One minor exception I would note is that if you're trying to balance gravity and bitterness, and you do decide to just let the "cards fall where they may" in terms of gravity, and it is non-trivially different, you might want to tweak your hop additions commensurately.

¹: "Relax, Don't Worry, Have a HomeBrew". :)

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Also, significant gravity differences may influence the amount of yeast you want to pitch for a good, healthy fermentation. –  baka Apr 16 '11 at 16:00
    
Good answer from point of view volume before/after boil, but at the risk of being a pedant, the gravity is a measure of the density of liquid when compared with water (1.000), not the ratio of water to sugar. An arbitrary example:If one had 1kg of water (1000 ml) and 1kg of sugar the ratio would be 1:1. The gravity would be approx 2.000 i.e. 1000g water + 1000g sugar (dissolved) divided by, a little over, 1000ml (due to addition of said sugar). –  iWeasel Apr 19 '11 at 15:50
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Here's a calculator on Onebeer. You'll need to know your expected preboil and postboil volume and your expected post boil gravity.

Yes, if it's too low, you can add extract. My preference is the lightest Dry Malt Extract I can find. If it's too high, you can add water, but make sure it's filtered/dechlorinated, etc.

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