Base malt, where nearly all the fermentable sugar in an all-grain recipe is derived, is easily identifiable as a light-colored malt that takes up a vast majority of the grain bill. The exception would be wheat or rye beers which can sometimes use equal parts pale and wheat/rye as the base malts. For example: 2 Row, 6 Row, Pale, Pilsner, Pale, Maris Otter, Vienna, etc.
Luckily, there are malt extract versions for nearly all types of base malts. If you are having trouble identifying the correct substitute talk to the staff at your local homebrew shop
You’ll likely notice a percent of efficiency listed on the all-grain recipe being converted. This percentage is how much of the potential sugars are extracted during the mash. A brewer who consistently hits 75% efficiency will extract more sugars and ultimately have a higher original gravity than a brewer with 65% efficiency while mashing the same amount of grains. Because of this, it is important to note the recipe’s efficiency when converting as it effects the amount of extract needed.
Liquid malt extract weighs more than dry malt extract, so there are different conversion rates for each. Based on the all-grain recipes assumed efficiency and the type of extract you prefer, use the following table from Ray Daniels’ Designing Great Beers to determine how much dry or liquid malt extract is needed per pound of base malt. It’s that easy.
If the recipe does not specify an efficiency, then either 70% efficiency or 75% efficiency are typical.
For example: Your recipe calls for 10 pounds of 2 Row malt and you are assuming 75% efficiency you will need 7.3 pounds of Liquid Malt Extract (10# x 0.73 = 7.3 pounds of LME)
For example: Your recipe calls for 10 pounds of 2 Row malt and you are assuming 75% efficiency you will need 6 pounds of Dry Malt Extract (10# x 0.60 = 6 pounds of DME)
Specialty grains are easily utilized by extract brewers to add more color and flavor variations. While you’re heating up the boil water, turn off the heat around 160-170°F and steep the specialty grains in a muslin bag for about half an hour. Then, simply pull the grain bag out, and return to heating the water for the boil. Just before the boil, stir in the extract and go about business as usual.
Converting malt extract recipes to base malt is the same process, but in reverse! You still need to identify your mash efficiency, which will determine the variable to use based on the information above. Simply divide the weight of the extract by the appropriate variable, and this will give you the amount of base grain needed to reach the same target original gravity. If you are converting all-extract recipes, keep in mind that you may need to consider using specialty grains if substituting for things like amber malt extract.
excerpt from American Hombrewers Association web page