How to Calculate Dough Hydration With a Sourdough Starter
Hydration is total water ÷ total flour × 100, and "total" includes the flour and water inside your starter. Split the starter by its hydration (a 100% starter is half flour, half water), add those to the dough's flour and water, then divide. 500 g flour + 350 g water + 100 g starter is 72.7% hydration, not the 70% you get if you ignore the starter.
The formula
starter flour = starter ÷ (1 + starter hydration ÷ 100)
starter water = starter − starter flour
hydration = (water + starter water) ÷ (flour + starter flour) × 100
Worked example
A common country loaf: 500 g flour, 350 g water, 100 g of starter fed 1:1 (100% hydration), 10 g salt.
- Starter: 100 ÷ 2 = 50 g flour and 50 g water
- Total flour: 500 + 50 = 550 g (this is 100%)
- Total water: 350 + 50 = 400 g
- Hydration: 400 ÷ 550 × 100 = 72.7%
- Starter: 100 ÷ 550 = 18.2% of the flour
- Salt: 10 ÷ 550 = 1.8%
Ignore the starter and you'd call this a 70% dough (350 ÷ 500) with 2% salt. That gap is why two bakers following "the same 70% recipe" can end up with doughs that handle very differently.
Stiff starters and other preferments
A starter fed at 50% hydration (100 g flour to 50 g water) holds more flour. 100 g of it is 66.7 g flour and 33.3 g water. The same rule covers poolish and biga: split each by its own hydration and add the parts to the totals.
| Starter | Dough water | Total flour | Hydration |
|---|---|---|---|
| None | 350 g | 500 g | 70.0% |
| 100 g at 100% | 350 g | 550 g | 72.7% |
| 100 g at 100% | 375 g | 550 g | 77.3% |
| 100 g at 50% | 330 g | 566.7 g | 64.1% |
Hitting a target hydration
To make a 75% dough with 100 g of 100% starter and 500 g total flour, the starter brings 50 g flour and 50 g water. You need 375 g water in total, so add 450 g flour and 325 g water to the starter. Check: 375 ÷ 500 = 75%.
Common mistakes
- Leaving the starter out. It's the most common reason a recipe feels wetter than its stated hydration.
- Assuming every starter is 100%. Check how yours is fed. A stiff starter changes the math.
- Counting salt or other ingredients as water. Milk, eggs and honey do add moisture, but they aren't water; note them separately if you track them.
- Comparing hydration across flours. Whole wheat and high-protein flours absorb more water, so 75% feels stiffer with them than with white flour.
Hydration is a number, not a recipe. Flour, temperature and your starter's strength all change how a dough at the same percentage behaves.
Questions
Does the starter count toward hydration?
Yes. A starter is flour and water, and both end up in the dough. Leaving it out understates the hydration, by about 2.7 points for a typical loaf with 100 g of starter.
How do I split a starter that isn't 100% hydration?
Flour = starter weight ÷ (1 + hydration ÷ 100). A 100 g starter at 50% hydration is 66.7 g flour and 33.3 g water.
Does salt count in hydration?
No. Hydration is water ÷ flour only. Salt is its own percentage, usually 1.8–2.2% of the flour.
Do I count poolish or biga the same way?
Yes. Any preferment is flour plus water. Split it by its own hydration and add both parts to the totals.