Sourdough starter hydration converter
Enter your starter's weight, its current hydration and the hydration you want. You see how much flour or water to add, step by step.
Result
Water to add
g
Flour to add
g
| Flour in your starter = weight ÷ (1 + hydration) | 50.0 g |
|---|---|
| Water in your starter = weight − flour | 50.0 g |
| Starter used | 100.0 g |
| Flour added | 12.5 g |
| Water added | 0.0 g |
| Flour in the new starter | 62.5 g |
| Water in the new starter | 50.0 g |
| New starter weight | 112.5 g |
| New hydration = water ÷ flour | 80.0 % |
How it works
Hydration is the weight of water divided by the weight of flour. The calculator splits your starter into flour and water, then adds only the ingredient needed to reach the new ratio. It never asks you to take flour or water out.
Formula
Flour in starter = M ÷ (1 + h1). Water in starter = M − flour. Wetter (h2 > h1): add water = flour × (h2 − h1). Stiffer (h2 < h1): new flour = water ÷ h2, add flour = new flour − flour. Same total weight: use U = M × (1 + h1) ÷ (1 + h2) of the starter when going wetter, or U = M × h2 × (1 + h1) ÷ ((1 + h2) × h1) when going stiffer, and top up to M.
Worked example
The calculator opens with this example; the result above is computed from it.
| Input | Value |
|---|---|
| Starter weight | 100 g |
| Current hydration | 100 % |
| Target hydration | 80 % |
| What to keep | Use all the starter (add flour or water) |
| Water to add | 0.0 g |
| Flour to add | 12.5 g |
100 g of starter at 100 % holds 50.0 g of flour and 50.0 g of water. To reach 80 %, add 0.0 g of water and 12.5 g of flour. You get 112.5 g of starter.
Assumptions and limits
- Hydration means water weight as a percentage of flour weight.
- All amounts are weights. Cups and spoons are not accurate enough for this.
- Nothing is ever removed: the calculator only adds flour or water, or tells you how much starter to set aside.
- The maths does not account for fermentation. A converted starter usually needs a feed or two to settle at its new hydration.
The result is an estimate for planning, not professional advice.
Sources
- wordloaf: rules for sourdough discard (flour = weight / (1 + hydration))retrieved 2026-10-12
- Calculover sourdough hydration (starter flour = starter weight / (1 + hydration/100)); calculator site that shows its formularetrieved 2026-10-12
- The Fresh Loaf forum thread, 2009-01-05 (comments 52805 Soundman, 52808 ryeaskrye): 180 g of 80 % starter + 45 g water = 225 g of 125 % starter; 225 g at 80 % = 125 g flour + 100 g waterretrieved 2026-10-12
Last updated:
Method in depth: Starter hydration maths: splitting a starter into flour and water
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