A rough estimate is multiply the base grain by .75 for LME and .6 for DME. This is assuming your recipe uses a basic malt like 2-row. If it uses a combination of base malts like 2-row and munich then the grain to extract substitution is difficult. In addition you may need to add specialty grains. Software like beersmith has a built-in converter so if you enter the all grain recipe it will convert to extract for you.
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