Converting commercial yeast recipes to sourdough

You found a recipe you want to try but it calls for commercial yeast. You want to use your carefully maintained sourdough starter. Read on for instructions on how to convert from one to the other. Pain au lait (picture below) was the first bread I baked after using this conversion process.

Some instructions ask to replace 1/2 cup of flour with the starter. This is not very accurate and will lead to problems if you are a novice baker. The hydration level of the starter could wreak havoc with the recipe. A high hydration starter could lead to a very wet dough and a low hydration starter could lead to a dry one. A newly fed starter will work very differently from an unfed one. To avoid these problems stick to weights.

Conversion process
You need to know % of hydration of your starter - given a quantity how much water and flour the starter contains.

Find the total amount of flour by weight used in your recipe. Calculate 40% of this amount (multiple by 0.4). That is be the amount of starter you need to use in your recipe.

Next calculate the amount of flour and water in grams you starter contains. For example, 100 grams of a 78% starter has 44 gm water to 56 gm flour. Reduce the flour in the recipe by the amount of flour your starter has. Reduce the amount of total liquid in your recipe by the amount of water your starter has.

You are all set to bake your favorite recipe with your own starter.