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...need more information... what kind of extract is included? What is targeted gravity for the 3.5? Are you also considering the hops (or is that figured out already).

The math is:

3.5 gal of (OG-1*1000) = (?)GU in 3.5 gal of wort --> GU-35

5 gal of (OG-1*1000) = (?)GU in 5 gal of wort --> GU-5

GU-5 minus GU-35 = GU difference

(GU difference)/(gravity pts. per lb. of malt) = Lbs. of malt needed.

I believe that is all right. I don't really feel like going back to check it too closely. Any takers?...
 
Yeah...

I've never bought the boxed kits, but from reading other people's posts, it seems like most kits include an info sheet that will give you expected OG - I'm not sure about IBU.

Two options, you could simply weigh the bag of extract they include and add 43% more extract or 3 lbs. (since 5 is 43% more than 3.5). Or...

The math:
Let's say the kit is supposed to have an OG of 1.090 at 3.5 gal

1.090 * 3.5 g = 315 BU

1.090 * 5 g = 450 BU <-- this is what you're shooting for

450 - 315 = 135 GU diff <-- so, this is what you have to make up with the addition

135 BU / 45 (ppg of DME) = 3 lbs. of DME <-- amount of extract needed to match OG with 5 gal batch

Like I said, you will need to adjust the hopping also. I'm not sure what volume you are boiling, but you could do roughly the same math and just bump up the amount of hops for each addition by a constant math. Unfortunately, the appropriate amount could be anywhere from 35% to 50%, depending on several factors (boil vol, wort gravity, etc.)
 
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