After this definition, we need to study the beer:
which are the color ranges (SRM)
alcohol content (ABV)
original gravity (OG)
final gravity (FG)
bitterness (IBU)
For that I would use the BJCP Style Guide (Beer Judge Certification Program), which is one of the most used in the world, and we will observe the information of the chosen style.
Let's make the classic beer: An American Pale Ale (APA). An APA has the following information:
Excerpt from the BJCP Style Guidelines - page 33
To calculate the quantity of malt use the following formula:
where TWM = Total Weight of Malt (in kg)
Assuming we are going to produce 20 liters of beer
TWM = 5 . 20 . [1,052 - 1,0] -> we used OG=1,052 (average value of 1,045 - 1,060)
TWM = 5,2 kg
We need 5,2 kg of malt to make a recipe of 20 liters of APA that will reach 1,052 of OG (original gravity)!
This is a simplified formula and uses a factor correction 5 to avoid more complex calculation of parameters like LDK (Litre Degrees per Kilogram) that some times is not available.
It is complicated, the variety is huge, and it depends on several factors, what color you want, flavor, etc.
So here are some tips:
Observe in BJCP or in conversations with other brewers which malts are most suitable for that recipe
Take into account the tradition and raw material of the region where the style was created
Always start with base malt, set a percentage for your recipe. Remember, you can use base malt up to 100%, so be very careful when using special malts, roasted, caramels… check the manufacturer's information on the maximum amount of this malt in a recipe (always in percentage).
Excerpt from the BJCP Style Guidelines - page 33
Traditionally and also indicated in the BJCP, we will use pale ale malt as a base.
We can define:
85% base pale ale malt
10% carahell, which will add body to the beer, give it a light caramelized sweetness and elevate the color.
5% crystal malt 110, which will give a golden shine and even stabilize the foam of the beer!
Let's create the recipe, just multiply the total weight of the malt by the corresponding percentage.