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Lets say you were aiming for 1.047 but only hit 1.032. In a 5 gallon batch, how much malt extract would you need to add to the fermenter to bring it up to the desired gravity?

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I like this question. If people read it, they may remember that it's OK to do this in a pinch. Wish I'd have know about it when I did this: brewadvice.com/questions/800/… – JackSmith Jun 16 '10 at 15:50
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I wouldn't add things to the fermenter normally. Its best to have corrected it in the kettle. See my comment about dilution in Pat's answer. In the kettle you could have just added DME directly. – brewchez Jun 16 '10 at 15:53
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+1 To Brewchez's comment on correcting gravity in the kettle. Pre-boil gravity will tell you exactly what your original gravity will be. Also a good idea to check it before each hop addition until you get the hang of what's going on. – Dean Brundage Jun 16 '10 at 17:35

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If we steal the numbers from this question, we can assume that liquid malt extract has a value of about 37 points per pound per gallon, and dry has about 44. This is not exact. 

total gravity points needed = Desired total gravity - current total gravity
total gravity points needed = 47 * 5 (desired reading * desired volume) - 37 * 5 (current reading * current volume)
total gravity points needed = 235 - 185
total gravity points needed = 50

When we talk about total gravity, we don't need to worry about the effect in volume. The amount of sugar in 50 gallons of water is the same as if you boil it down to 1 gallon. It's total gravity, or total sugar content, essentially.

So, you need to add 50 points to the batch.

Dry = 50 / 44 = 1.14 lbs
Liquid = 50 / 37 = 1.35 lbs

I think my math is right. But it's early :) The numbers will honestly depend on the manufacturer of your malt, but you can plug in those numbers instead.

Let me know if that all makes sense.

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Great, except the 37 should be a 32, right? That makes it 75 points, right? 1.14 lbs dry or 2.02 lbs liquid. – Nicholas Trandem Jun 16 '10 at 15:57
But how are you going to get that DME into the fermentor? You'll have to add it to water and boil it. You need to account for the volume in that step. The rest of your calcs and assumptions are right. – brewchez Jun 16 '10 at 15:57
Great, except the 37 should be a 32, right? That makes it 75 points, right? 1.70 lbs dry or 2.02 lbs liquid – Nicholas Trandem Jun 16 '10 at 15:59
@brewchez Ah I misread. I was thinking during the boil. So ya, volume changes a bit – hookedonwinter Jun 16 '10 at 16:02
@Joe Right. Whatever number your extract is, use that :) – hookedonwinter Jun 16 '10 at 16:03
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In my experience:

1 lb of Dry extract will add 44 points per gallon (potential = 1.044), or 8.8 points in 5 gallons.

47-32 = 15 points needed to get where you want to be.

15/8.8 ~ 1.7 lb of DME.

Make a mini-wort - boil and cool the DME in as little water as possible and then add it to the fermenter slowly to minimalize the splash factor.

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Ah! but if you make a mini wort you are diluting the points PER GALLON! So you need to account for the % increase in volume too. – brewchez Jun 16 '10 at 15:52
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Y'know, I doubt it really matters if it's perfect down to the third decimal point. Why not just boil 2 lbs of DME in a liter or so of water, throw it in and see what happens? Isn't that close enough? Does it -really- need to be plus or minus .001 of an exact gravity?

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