Split the starter into flour and water first
Starter hydration is the weight of water divided by the weight of flour, multiplied by 100. Equal weights of flour and water give 100% hydration. A stiff starter at 60% hydration contains 60 g water for every 100 g flour. A scale tells you the weight of the combined starter, so work backwards to find the two components. [2]
Starter flour = starter weight ÷ (1 + starter hydration ÷ 100).
Starter water = starter weight − starter flour.
Total hydration (%) = (added water + starter water) ÷ (added flour + starter flour) × 100.
Use the same weight units throughout. Enter the starter hydration as 100 for an equal-weight starter or 60 for a 60% starter. Count only the starter that goes into the dough. The part left in the mother jar has no effect on this calculation.
Two examples with the same 120 g of starter
| Ingredient or calculation | Liquid starter at 100% | Stiff starter at 60% |
|---|---|---|
| Added flour and water | 500 g flour and 350 g water | 500 g flour and 350 g water |
| Flour in 120 g starter | 120 ÷ 2 = 60 g | 120 ÷ 1.6 = 75 g |
| Water in 120 g starter | 120 − 60 = 60 g | 120 − 75 = 45 g |
| Total flour and water | 560 g flour and 410 g water | 575 g flour and 395 g water |
| Total hydration | 410 ÷ 560 × 100 = 73.2% | 395 ÷ 575 × 100 = 68.7% |
Both columns have the same added-water percentage of 70%. The liquid starter increases total hydration above that figure, while the stiffer starter brings it below. Replacing starter by weight can therefore change a dough even if the added water and flour never move.
Check what the recipe already counts
A recipe card may show a final-dough mix with a separate levain line. In that case, add the levain’s flour and water once. A total formula may already include flour and water from the levain. In that case, divide the stated totals directly. If the recipe has a separate levain build, use the hydration of the portion that actually goes into the dough, especially if it differs from the mother jar. [2]
Check your feed notes if the starter hydration is uncertain. The ratio of your last flour and water additions is a useful clue, but the old seed also contributes flour and water. For a new build made from a large proportion of seed, include its contribution. If you lack those weights, label the result an estimate.
Crispy Crust’s planned recipe tools would keep the starter hydration beside the ingredient weights. That would help you compare formulas with the same denominator and catch a change when you switch from a liquid starter to a stiff one.
Add up more than one preferment
If you add two builds, split each at its own hydration and sum their flour and water separately. For example, 100 g of equal-weight starter gives 50 g of each, while 80 g at 60% hydration gives 50 g flour and 30 g water. Together they contribute 100 g flour and 80 g water. Keep those lines separate in your notes so you can audit the total.
Check the sourdough hydration calculator to include the flour and water in your starter without counting them twice.