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Sourdough Starter Calculator
A levain build is the feed you give your starter the night before (or morning of) a bake so you have exactly the amount of ripe, active culture the recipe calls for. Enter how many grams of levain you need, how much ripe starter you actually have in the jar, your hydration, flour type, and kitchen temperature. The calculator returns the exact grams of starter, flour, and water to mix, the feed ratio, and an estimated time to peak. When the feed ratio would exceed 1:5 it automatically plans a healthier two-step build instead.
Common levain builds at a glance
All rows below are for 300 g of levain at 100% hydration with white flour in a 22 C (72 F) kitchen. Bigger ratios stretch the food supply, so they take longer to peak; that is how you schedule an overnight build versus a same-morning one.
| Feed ratio | Starter | Flour | Water | Peak (approx.) |
|---|---|---|---|---|
| 1:1:1 | 100 g | 100 g | 100 g | 5 h |
| 1:2:2 | 60 g | 120 g | 120 g | 7 h |
| 1:4:4 | 33 g | 133 g | 133 g | 10.5 h |
| 1:5:5 | 27 g | 136 g | 136 g | 12.5 h |
Whole-grain flour ferments about 25% faster (multiply times by 0.76) and mixed flour about 14% faster (0.86). A 26 C kitchen cuts the times by roughly 16%; an 18 C one adds roughly 68%.
How the math works
The calculator first works out the feed. The total build equals the levain you need; subtracting the starter you have on hand gives the feed, which splits between flour and water by hydration: flour = feed / (1 + hydration / 100) and water is the rest. If you already have more ripe starter than the build needs, it uses 20% of the build as seed and treats the mix as a simple refresh. The feed ratio is flour divided by starter.
Time to peak starts from a base of 2.5 + 1.6 x ratio hours (ratio capped at 12), then multiplies three environment factors: temperature 2^((24 - temp C) / 8), meaning fermentation speed doubles per 8 C of warmth; hydration 1 + (100 - hydration) x 0.005, since stiffer levains move slower; and a flour factor of 1.0 for white, 0.86 for mixed, and 0.76 for whole grain. The usable peak window runs from that estimate to about 20% past it.
When the ratio exceeds 1:5, one big feed dilutes the yeast too much, so the calculator splits the build in two. The intermediate size is the geometric mean of your seed and the total build, which makes both feeds land at roughly the same comfortable ratio; each step gets its own peak time and you feed the second as soon as the first peaks.
Frequently asked questions
How much starter, flour, and water do I need for 300 g of levain?
With 60 g of ripe starter on hand and 100% hydration, mix 60 g starter, 120 g flour, and 120 g water. That is a 1:2:2 feed, and with white flour at 22 C it peaks in roughly 6.5 to 8 hours. With less starter on hand the ratio grows and the peak takes longer.
Why does the calculator sometimes show a two-step build?
When the flour-to-starter ratio goes above 1:5, a single feed dilutes the yeast so much that fermentation gets weak and slow. Splitting into two smaller feeds, each taken to its peak, keeps every feed in a healthy 1:1 to 1:5 range and produces a stronger levain, usually in less total time.
How does temperature change the time to peak?
Fermentation speed roughly doubles for every 8 C of warmth. The calculator uses a factor of 2^((24 - temp) / 8): a 1:2:2 white-flour levain that peaks in about 5 hours at 26 C takes about 7 hours at 22 C and over 9 hours at 18 C.
Never miss the peak
The Sourdoughy app runs this same calculator, then reminds you when your levain should be peaking and checks a photo of the jar with AI to confirm it is actually ready to mix.
Download Sourdoughy free