Certain repeated questions turn up from time to time that are valuable, but the replies required are lengthy. This post was written with the thought that it can be linked for future replies to the question of Levain and how it is calculated.
There are 2 main methods used to calculate the percentages of Levain used in a formula.
Total levain weight divided by the flour weight of the final dough
Levain flour divided by TOTAL flour used in the recipe
The various methods of calculating Levain can be confusing. Realize that many highly respected bakers use either of these methods.
Method #1 - Total levain weight (including the water and flour) divided by the flour weight of the final dough
This is best explained as the Levain being considered an ingredient according to Bakers Percentages, the same way salt would be calculated in a formula. It is important to realize that the weights of both the water and flour in the Levain are not part of the Total Water and Toal Flour in the formula.
Flour = 1000g (100% Bakers Percentage)
Water = 700g (70%)
Salt = 20g (2%)
Levain = 240g (24%) - Since the hydration of the Levain is not specified we commonly assume 100% hydration
Total Dough Weight = 1960g
Hydration = (Total Water = 700+120=820 ) + (Total Flour = 1000+120=1120) 820/1120=73.2% Hydration
Method #2 - Levain flour divided by TOTAL flour (including the flour in the Levain) used in the formula
This method is called the Percentage of Pre-Fermented Flour (PFF). It is this method that has been adopted as the standard by the Bread Bakers Guild of America (BBGA). Most bakers on The Fresh Loaf use this one.
Total Flour = 1000g (100%)
Total Water = 700g (70%)
Salt = 20g (2%)
PPF (Pre-Ferment) = 12% Levain Hydration = 100% - With this method the hydration of the Levain is always specified. In this example there is 120g water and 120g flour in the levain.
Total Dough Weight = 1720g
Hydration = (Total Water 700g) + (Total Flour 1000g) 700/1000= 70% Hydration
NOTE - the dough minus the Levain contains 580g Water and 880g Flour.
Since this post was published as a reference for future questions, please let me know if you find errors or have additional pertinent information.