It's simple. DME gives you SG = 1.046 lb/g, i.e. 46 points. Say you want to have 5 gallons wort with gravity 1.050, or 50 points. Here is what you need to do: 5 * 50 = 250 Points. Next, 250 / 46 = 5.44 lb. If you are going to use LME - it gives you 37 points instead of 46. Now you can dance from here.
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