Serious bakers use weights in their recipes, because scooping technique and
humidity introduce a lot of variance in volume-based measurements. To make it
easy to scale recipes up and down, bakers often express weights as percentages
of the flour weight. This is completely different from normal math, where we’d
use percentages of the *total* weight.

There are two approaches to percentage-based sourdough baking: one breaks the starter into flour and water weights, and the other treats it as a separate ingredient (ignoring the flour content). As an example, consider a mix of 100g flour, 100g water, 10g salt, and 10g starter (itself an even mix of flour and water, aka 100% hydration). Using the first approach, we’d calculate percentages like this:

Ingredient | Weight | Baker’s Percentage | Normal Percentage |
---|---|---|---|

Flour | 100g dry + 5g in starter | 100 | 47.7 |

Water | 100g + 5g in starter | 100 | 47.7 |

Salt | 10g | 9.5 | 4.6 |

Using the second approach, we’d have this:

Ingredient | Weight | Baker’s Percentage | Normal Percentage |
---|---|---|---|

Flour | 100g | 100 | 45 |

Water | 100g | 100 | 45 |

Salt | 10g | 10 | 5 |

Starter | 10g | 10 | 5 |

The Tartine bread book uses the second approach. Chad Robertson says that it’s more common in commercial bakeries: because the same starter is used in all the recipes, it’s easier to treat it as an ingredient rather than a mix of flour and water. I do the same.