Starter Hydration Converter: Change a 100% Starter to 50%, 75% or 125%
Starter hydration is the water in the starter as a percent of its flour. A starter fed equal weights of flour and water is 100%. To change it, you keep a little of the old starter and feed it flour and water in a different proportion. The calculator finds the water to add for the flour you choose.
The math
A starter at hydration H% splits into flour = starter × 100 ÷ (100 + H) and water = starter × H ÷ (100 + H). Keep K grams, add F grams of flour and W grams of water, and the new hydration is
(water in kept starter + W) ÷ (flour in kept starter + F) × 100
To hit a target hydration T% with the flour you chose, solve for the water:
W = T ÷ 100 × (flour in kept starter + F) − water in kept starter
If W comes out negative, the old starter is already too wet for the target at that flour weight. Add more flour instead.
Worked example
Keep 50 g of a 100% starter. It holds 50 × 100 ÷ 200 = 25 g flour and 25 g water. Add 50 g flour, so the flour totals 75 g.
| Target hydration | Water to add | New starter | Check |
|---|---|---|---|
| 50% | 12.5 g | 112.5 g | 37.5 ÷ 75 = 50% |
| 75% | 31.25 g | 131.25 g | 56.25 ÷ 75 = 75% |
| 100% | 50 g | 150 g | 75 ÷ 75 = 100% |
| 125% | 68.75 g | 168.75 g | 93.75 ÷ 75 = 125% |
What it does to your dough
The starter's hydration changes the true hydration of the dough it goes into. With 500 g flour, 350 g water and 100 g of starter, the hydration calculator gives 67.6% true hydration with a 50% starter, 72.7% with a 100% starter and 74.5% with a 125% starter. The recipe says 70% in all three. When you change your starter, recheck the dough's true hydration.
How fast the new starter rises depends on your flour, temperature and habits, and this page gives no time for it. Time it yourself and scale with the schedule calculator.
Sources
- Baker's percentage arithmetic: hydration = water ÷ flour × 100, applied to the flour and water in the starter. All numbers above are computed with it.