Ask how much starter goes into a sourdough loaf and you can get answers from 5% to 50%, sometimes for the same recipe. Much of that spread is not disagreement but bookkeeping: people count the starter in different ways. This guide uses one convention, says what it is, and shows what the amount changes: the bulk time at your temperature and the dough's real hydration.
How the calculator counts
The amounts are in baker's percentages: every ingredient as a share of the flour weight. In the bake schedule calculator, the starter amount is the weight of ripe levain divided by the weight of the flour you add, with the levain at 100% hydration (equal weights of flour and water). Levain here simply means starter that has been fed and has risen to its peak, ready to mix; many recipes just call it starter. So:
- 500 g of flour with 100 g of levain is 20%.
- The levain's own 50 g of flour is not added to the 500 g when the percentage is worked out.
Other recipes count differently. Some divide by all the flour, including the levain's (the pre-fermented flour). Others give the starter as a share of the dough weight, or quote the small amount of mature starter that went into a levain build rather than the levain itself. The same loaf can therefore be written as several different percentages. When a recipe's numbers look odd, check which of these it uses before changing anything.
What the amount does to bulk
More starter means more yeast and bacteria at the start, so the dough reaches the end of bulk sooner (about 50–75% bigger, domed, bubbles at the sides). Less starter takes longer, but not in proportion: in the calculator's model, half the starter makes bulk about a third longer, not twice as long. This is based on The Sourdough Journey's controlled series of test doughs (bulk fermentation timetables), where going from 20% to 10% starter added about a third to the bulk time. Their absolute times are longer than ours, because flour, starter and the end point of bulk differ between kitchens; the model borrows only the ratio between their times, not the hours themselves. Their series covers 5–20% starter at 19–23°C, so the 25% and 30% rows below are extrapolated.
| Levain, % of the flour you add | Levain for 500 g flour | Bulk at 20°C (68°F) | Bulk at 22°C (72°F) | Bulk at 24°C (75°F) | True hydration at 75% water |
|---|---|---|---|---|---|
| 10% | 50 g | about 9¾ h | about 8 h | about 6½ h | 76.2% |
| 15% | 75 g | about 8¼ h | about 6¾ h | about 5½ h | 76.7% |
| 20% | 100 g | about 7½ h | about 6 h | about 5 h | 77.3% |
| 25% | 125 g | about 6¾ h | about 5½ h | about 4½ h | 77.8% |
| 30% | 150 g | about 6¼ h | about 5¼ h | about 4¼ h | 78.3% |
At 22°C (72°F) the model predicts about 8 h of bulk with 10% levain, 6 h with 20% and 5¼ h with 30%. These are the model's predictions; your starter's strength and your flour shift them, and the dough's signs have the final say. Notice that temperature moves bulk far more than the starter does: 20% at 20°C (68°F) takes longer than 10% at 24°C (75°F).
The hidden hydration
A 100%-hydration levain brings its own water as well as its own flour, so it nudges the dough wetter than the recipe's hydration says. With 20% levain, a recipe written at 75% water is really about 77.3% hydration, and one written at 70% is about 72.7%. The last column of the table above shows the effect at 75% water. That is plain arithmetic, not a model, and it assumes the recipe's hydration counts only the flour and water you add. If a recipe already includes the levain in its hydration, the figure it gives is the real one. The hydration guide explains what a few points of hydration feel like in the hand. To convert a yeast recipe and work out the water to hold back, see convert a yeast recipe to sourdough.
Building the levain
To get 100 g of ripe levain for a loaf made with 500 g of flour, mix 25 g of starter with 50 g of flour and 50 g of water (a 1:2:2 feed) and let it rise to its peak. The calculator's starter model puts that peak at about 5½ h at 22°C (72°F); the starter feeding ratio guide has the times for other ratios and temperatures. Start it that long before you plan to mix, with starter that was fed within the last day rather than straight from the fridge. Use 100 g in the dough; the 25 g left over can go back into your starter jar or be discarded.
Choosing an amount
Pick the amount that fits the time you have at the temperature you have:
- 20% is the calculator's default and a common middle ground: a bulk that fits into a normal day in a 22–24°C (72–75°F) kitchen.
- Less starter stretches bulk by a few hours: useful for a warm kitchen or a long day away; for a bulk while you sleep, see overnight bulk fermentation. In real heat it is not enough on its own; summer sourdough covers cooling the dough as well.
- More starter shortens it a little: useful in a cold kitchen when you are short on time, though a warmer spot does more. Winter sourdough covers the cold side.
Whatever the amount, the starter should be at its peak when you mix. If it is not getting there, see sourdough starter not doubling.
Common questions
Is 20% starter the same as 100 g?
Only for 500 g of flour. The percentage is of the flour you add, so 20% is 100 g for 500 g of flour, 200 g for 1 kg.
Can I use starter straight from the fridge?
Not as levain. Feed it and let it rise to its peak first; see the fridge section of sourdough starter not doubling.
Try it with your numbers
Put your flour weight, starter amount and kitchen temperature into the bake schedule calculator. It shows the grams, the true hydration and a schedule with the bulk time for your kitchen. For how temperature moves bulk time on its own, see bulk fermentation by temperature.