Sourdough Hydration Calculator (Including the Starter)
Hydration is the water in a dough as a percentage of its flour. In sourdough there is a catch: the starter is itself flour and water, so a recipe that says "70% water" can really be a wetter dough once the starter is counted. This calculator gives both numbers.
The math
A starter at hydration h% splits like this:
- flour in starter = starter weight × 100 ÷ (100 + h)
- water in starter = starter weight × h ÷ (100 + h)
Then:
true hydration = (water added + water in starter) ÷ (flour added + flour in starter) × 100
Worked example
500 g flour, 350 g water and 100 g of a 100% starter:
- The starter holds 100 × 100 ÷ 200 = 50 g flour and 50 g water.
- Total flour = 500 + 50 = 550 g. Total water = 350 + 50 = 400 g.
- True hydration = 400 ÷ 550 = 72.7%, against 70% if you ignore the starter.
With a stiffer 50% starter, the same 100 g holds 66.7 g flour and 33.3 g water, and the true hydration is 383.3 ÷ 566.7 = 67.6%: drier than the recipe number, not wetter.
Quick table
All rows use 500 g flour and 100 g starter.
| Water added | Starter hydration | Recipe hydration | True hydration |
|---|---|---|---|
| 325 g | 100% | 65% | 68.2% |
| 350 g | 100% | 70% | 72.7% |
| 375 g | 100% | 75% | 77.3% |
| 350 g | 50% | 70% | 67.6% |
| 350 g | 125% | 70% | 74.5% |
When it matters
The bigger the share of starter, the more the two numbers differ. With 500 g flour, 350 g water and a 100% starter: 25 g of starter gives a true hydration of 70.7%, while 200 g gives 75.0%, both for a recipe that says 70%. If you're comparing two recipes, compare their true hydrations.
Sources
- Formulas: hydration = water ÷ flour × 100, applied to the flour and water in both the bowl and the starter. All numbers above are computed with them.