Calculator
Convert a yeast recipe to sourdough
Hydration stays all the water divided by all the flour. The starter replaces the commercial yeast and already contains some of that flour and water. The yeast figure underneath is a separate comparison. It is not a substitute for the starter.
Quick answer. A 1000 g flour dough at 70% hydration, moved to 20% starter at 100% hydration, uses 900 g flour, 600 g water, and 200 g starter. True hydration stays 70%. Commercial yeast is left out.
Sourdough formula
200 g starter · true hydration 70%
| Flour to add | 900 g |
|---|---|
| Water to add | 600 g |
| Salt | 20 g |
| Starter | 200 g |
The starter already holds 100 g flour and 100 g water. Commercial yeast is not added.
If it stayed a yeast dough
0.5 g instant yeast · 0.05% of flour
The commercial-yeast model, for 1,000 g of flour. Not a weight of starter, and not a substitute for the starter above.
Timing, only where someone measured it
At 74°F (23°C), in a dough of 90% King Arthur bread flour, 10% whole wheat, and 2% salt, with starter fed 1:2:2: 20% starter reached a 75% rise in 8.5 hours and a 100% rise in 9.5 hours. At 10% starter the timetable lists 12.5 hours.
For a typical sourdough bread recipe, the author lets bulk fermentation play out at room temperature over 3 to 5 hours, and says the period is dictated by the bread, the desired dough temperature, and how warm the dough is kept.
Those ranges belong to the doughs in the sources. This page does not turn them into a timer for your formula.
The default comparison is 0.5 g instant yeast (0.05% for 12 hours at 21.1°C). That is not the 200 g of starter.
Questions
- How do I convert a yeast recipe to sourdough?
- Keep the hydration, which is all the water divided by all the flour. Replace the commercial yeast with starter. At the default 20% inoculation and 100% starter hydration, 1000 g of flour uses 200 g of starter, which already contains 100 g of flour and 100 g of water. You add 900 g of flour and 600 g of water if the target is 70% hydration. True hydration stays 70%.
- What does 20% starter mean?
- On this site, inoculation is starter weight divided by total flour. Twenty percent is the default on this page, not a rule. Change it. Prefermented flour is only the flour inside the starter. At 100% starter hydration that is half the inoculation.
- Can I turn sourdough back into a yeast quantity?
- The reverse mode runs the existing commercial-yeast model on the flour, for the hours and dough temperature you enter. It is not a claim that the starter equals that yeast weight. A starter does not convert to grams of instant yeast in any reliable way. The number is what the yeast calculator would say if the leaven were commercial yeast instead.
- How long should sourdough bulk fermentation take?
- Only the cited ranges are on this page. The Sourdough Journey’s December 2022 timetable, for a specific 90% bread-flour dough at 74°F (23°C), lists 8.5 hours to a 75% rise and 9.5 hours to a 100% rise at 20% starter, and 12.5 hours at 10% starter. King Arthur’s bulk-fermentation article says the author lets a typical sourdough go 3 to 5 hours at room temperature, and that the dough dictates the time. Your dough is not those tests.
- Does oil in the recipe count as hydration?
- No. Hydration stays water only. Oil and sugar stay their own percentages of the flour when the recipe moves to starter. The yeast percentage is dropped, because the starter is the leaven.
Last reviewed 1 November 2026.
Sources
Hydration is all water divided by all flour, including the starter. The 20% inoculation and 70% hydration are this page’s example. The reverse yeast figure uses the commercial-yeast model and is not a starter equivalent. Timing is only the ranges those two sources published.