A starter’s hydration expresses its water mass relative to its flour mass. A 100% hydration starter contains equal masses of flour and water; it does not mean that the starter is entirely water.
When a formula includes starter, separate those two components before calculating total flour and total water. Keep the actual starter as one ingredient to weigh, while using its components for the overall calculation.
The equal-parts example
For 200 g of starter at 100% hydration, the total is split into 100 g of flour and 100 g of water. The two components add back to the 200 g you will use.
This split depends on the hydration you enter. It is not a default that can safely be applied to every starter. If you do not know the hydration, preserve the starter mass and resolve that missing information before relying on an overall hydration result.
The same calculation works at other hydrations
Write the hydration as a percentage H. The flour mass is starter mass × 100 ÷ (100 + H). The water mass is starter mass × H ÷ (100 + H). The two formulas use the same denominator and together recover the original starter mass.
For an illustrative 200 g starter at 50% hydration, the split is approximately 133.333 g flour and 66.667 g water. The displayed figures may be rounded, but the calculation should retain the underlying values.
| Declared hydration | Flour contribution | Water contribution |
|---|---|---|
| 100% | 100 g | 100 g |
| 50% | Approximately 133.333 g | Approximately 66.667 g |
Add each contribution to the right total
Consider a formula with 500 g of added flour, 350 g of added water, 200 g of 100% hydration starter, and 12 g of salt. The starter adds 100 g flour and 100 g water. Total flour is therefore 600 g, and total water is 450 g.
The overall hydration is 450 ÷ 600 × 100, or 75%. Salt is 12 ÷ 600 × 100, or 2%. The denominator includes the starter’s flour, while the water total includes its water. Putting starter water in the flour base would give the wrong result.
Count the starter once in the dough weight
The dough weighs 500 + 350 + 200 + 12 = 1,062 g. You can also express that as 600 g total flour + 450 g total water + 12 g salt. These are two views of the same ingredients.
Do not add the 200 g starter again after adding its components to the totals. A clear workbench separates the amounts you add from the overall accounting, so it is apparent which view each number belongs to.
Resize without changing the proportions
If the target dough weight is 1,593 g, the multiplier for this formula is 1.5. The amounts to add become 750 g flour, 525 g water, 300 g starter, and 18 g salt.
The larger starter contributes 150 g flour and 150 g water. Total flour becomes 900 g and total water 675 g, retaining 75% hydration. The ingredient weights add to 1,593 g. This checks both the serving of starter and the overall arithmetic.
Keep the declared hydration with the recipe
Save the starter mass and hydration together. If you change either one later, recompute the overall formula from the original ingredient values. A note such as “starter” without its hydration is not enough to reproduce the split.
These calculations describe mass proportions. They do not predict starter activity, fermentation time, or when a dough is ready. Keep that practical guidance with the original recipe rather than inferring it from the percentage table.