Formula
Every ingredient is a percent of total flour, and total flour includes the flour in the starter. That is baker’s percentage as King Arthur and The Perfect Loaf keep it.
starter = total flour × starter %
starter flour = starter ÷ (1 + starter hydration)
added flour = total flour − starter flour
water to pour = hydration × total flour − water in the starter
salt = total flour × salt %
Weights are grams. A 100% hydration starter is equal parts flour and water by weight.
Worked examples
500 g flour, 75% water, 20% starter at 100% hydration, 2% salt. Starter 100 g, of which 50 g is flour and 50 g is water. Added flour 450 g. Added water 325 g. Salt 10 g. Dough 885 g.
The same loaf scaled from 100 g of starter. Total flour is 100 / 0.20 = 500 g. The weights do not change.
Reference
500 grams of total flour. Starter is 100% hydration. “Water” is what you pour, after the starter’s water is counted.
| Hydration | Starter | Salt | Flour | Water | Starter | Salt |
|---|---|---|---|---|---|---|
| 70% | 20% | 2% | 450 g | 300 g | 100 g | 10 g |
| 75% | 20% | 2% | 450 g | 325 g | 100 g | 10 g |
| 80% | 15% | 2% | 463 g | 363 g | 75 g | 10 g |
| 85% | 15% | 2.2% | 463 g | 388 g | 75 g | 11 g |
Questions
How does a sourdough hydration calculator treat the starter?
Total flour is 100%, and the flour inside the starter counts. A 100% hydration starter is half flour and half water. On 500 grams of flour at 20% starter, that is 100 grams of starter: 50 grams of flour and 50 grams of water. The flour you add is 450 grams. At 75% hydration the dough wants 375 grams of water, so you pour 325 grams. Salt at 2% is 10 grams. The dough weighs 885 grams.
What is a sourdough starter calculator, as opposed to a bread calculator?
Same baker’s math, started from the other end. Pick “scale from starter” and type the jar you already have. A 100 gram starter at 20% implies 500 grams of total flour, and the rest of the formula is unchanged.
What if my starter is not 100% hydration?
Type its hydration. Starter flour = starter ÷ (1 + hydration). A stiff 50% starter is two parts flour to one part water. Leave it at 100 if you feed equal weights.
Why did the water come out negative?
The starter already holds more water than the dough hydration you asked for. Raise the hydration or lower the starter percentage. The same warning appears if the starter’s flour is more than the total flour.
Sources
- Baker’s percentages: total flour, including flour in the preferment, is 100%. King Arthur Baking and The Perfect Loaf both keep the starter’s flour and water inside that 100%.