Converting all-grain recipes to extract recipes can be accomplished in 3 steps. Note: converting recipes from extract to all-grain simply reverses the calculations as shown in Converting Extract Recipes to All-Grain Recipes, found below.
Extract recipes make use of either dry (DME) or liquid (LME) malt extract. DME is 4% water by weight, while LME is 20% water by weight, so less DME by weight is needed than LME for the same gravity as reflected in the conversion table below.
Base malt is the source of nearly all the fermentable sugars (saccharides) in an all-grain recipe, usually a light colored barley malt that makes up the majority of the grain bill. An exception is wheat or rye beers that can use equal parts pale barley malt and wheat malt or rye malt as the base malts. Base malts usually comprise 70% or more of a grain bill.
The recipe may indicate the percent of efficiency (BE) of potential sugars that are extracted during the mash. This efficiency affects the amount of malt extract needed. If no percent efficiency (BE) is given, assuming 75% efficiency gets us in the ballpark.
For example, if a 1 gallon recipe calls for 1.5 lbs of base malt with extract potential of 1.038 ppg, then the expected gravity from mashing is 1.5 lbs x 38 points x 75% BE = 42.75 points = 1.043 ppg. Thus, our target gravity from the malt extract should be the same (43 points).
Most dry malt extracts (DME) deliver 1.043 extract potential (EP) per pound (43 points), so divide the target gravity (43 points) by DME's extract potential (43 points) = 1.0 lb of DME = 1.5 lb of base malt.
Most liquid malt extracts (LME) deliver 1.036 extract potential (EP) per pound (36 points), so divide the target gravity (43 points) by LME's extract potential (36 points) = 1.2 lb of LME = 1.5 lb of base malt.
Most recipes include specialty grains in small quantities for color, aroma, flavor, mouthfeel, etc. These specialty grains (quantities given in the recipe) can be steeped for 30 minutes at 150 to 155F in a muslin bag. Remove the bag from the pot before the start of boil.
First, determine the amount of gravity expected from the malt extract in the recipe. That is the target gravity for the base malts of the all-grain recipe.
Second, determine the amount of base malts needed to meet the target gravity, using the following formula. (Target Gravity / EP) / BE = Pounds of Base Malt.
For example, given a desired target gravity of 43 points, a base malt with EP of 38 points, and BE of 75%:
(43 Target Gravity / 38 EP) / 0.75 BE = 1.5 lbs Base Malt.
Third, all specialty grains should be added to the base malts in the mash, and should be taken into account for determining OG. Though their gravity contributions will be small, given sufficient diastatic power (DP), they all add up. See Calculating Diastatic Power.
Additionally, take into account any specialty grains used within amber and dark malt extracts. Consult the maltsters for the malt makeup of their malt extracts.