Does batch size affect bulk fermentation time? In the calculator's model, not directly: a dough at a given temperature with a given levain percentage takes the same time whether it is 500 g or 4 kg. The batch size enters the model in one place only, through how fast the dough warms or cools toward the room, and that moves bulk by about 45 minutes between a small and a large home batch in the example below. Bakers describe a larger effect for bakery-size batches, and the model leaves out the reason. This guide gives what the model says, what two bakers' forum threads say, and what to do with a batch bigger than usual.
What the model says
The bulk time in the bake schedule calculator comes from the dough temperature and the levain percentage (bulk fermentation by temperature): in a 21°C (70°F) kitchen with 20% levain the model predicts about 6¾ h, for any amount of dough. Fermentation is a rate per gram, and twice the dough has twice the yeast and bacteria working on twice the flour, so the percentage of levain, not the number of grams, sets the pace.
Batch size comes into play only when the dough is not at room temperature at the start of bulk, which is the usual case: warm water and mixing leave it a few degrees warmer, cold flour or a cold bowl a few degrees cooler. The calculator's optional field "Dough temperature after mixing" models the dough drifting toward the room while it ferments, and the speed of that drift depends on the mass. A big batch holds its heat longer, because it has less surface for its weight. The model puts the drift's time constant (roughly, how long the dough takes to close most of the gap to the room) at 2 hours for a 1 kg batch, growing with the cube root of the mass and kept between 1 and 3 hours. That is an estimate from the physics of a lump of dough, not a measurement; the calculator shows a range for that reason. The table gives the model's bulk for a dough mixed warm, at 27°C (81°F), in a 21°C (70°F) room, with 20% levain:
| Batch (total dough weight) | Cooling time constant the model uses | Bulk, dough at 27°C (81°F) in a 21°C (70°F) room | Model's range |
|---|---|---|---|
| 500 g | 1.6 h | about 5¾ h | 5¼–6¼ h |
| 1 kg | 2.0 h | about 5½ h | 5–6 h |
| 2 kg | 2.5 h | about 5¼ h | 4¾–5¾ h |
| 4 kg | 3.0 h | about 5 h | 4½–5¾ h |
A 500 g batch (one small loaf) cools to the room within a few hours and the model predicts about 5¾ h of bulk; a 4 kg batch (four or five large loaves) stays warm longer and gets about 5 h. The difference is about 45 minutes. Both are far from the 6¾ h of a dough at room temperature from the start and the 3¾ h of one held at 27°C (81°F) throughout. The effect runs the other way for a cold dough in a warm room: 21°C (70°F) dough in a 27°C (81°F) kitchen takes about 4½ h at 500 g and about 5 h at 4 kg, because the big batch is slower to warm up. The calculator takes the mass from your recipe's total dough weight, so a bigger batch already gets the longer time constant when you enter the dough temperature. The time constant stops growing at 3 hours, which the model reaches at about 3.4 kg of dough, so it treats every batch above that alike.
In the model, then, batch size matters only while the dough and the room differ in temperature, and the shift is well under an hour for home batches.
What bakers report
Bakers' reports are mixed at home sizes and point to a real effect only at bakery sizes. Two threads on The Fresh Loaf lay out the range:
- In Batch Size and Dough Performance: Correlated? (2017), a baker who usually mixed about 480 g of dough found that a 960 g batch of the same recipe handled better, though it took longer in bulk, and asked why. One reply said large masses do ferment differently, which bakers call the mass effect, but did not expect a meaningful difference between 480 g and 1–2 kg; another saw no difference at all scaling a recipe from one loaf to a dozen. The thread ended without a consensus; the original poster wondered whether handling the small batches explained it and planned to test.
- In Mass Effect (2024), the starting point was a poster's reading of Hamelman's Bread: that larger batches ferment faster, and that a home baker scaling down might use about 25% more mature culture because the small mass ripens sluggishly. The poster could not find the passage again, and the thread did not confirm it, so take it as quoted, not checked. The explanations offered were the heat that fermentation itself gives off, which a large mass loses more slowly, and the smaller surface for its weight. One poster put the point where it becomes significant at roughly 4.5–9 kg (10–20 lb) and up, which is why bakeries chill the water and adjust the levain for large batches, and called it negligible at home-mixer sizes.
In the model's terms, 25% more mature culture is a bigger seed in the levain build, and by the starter rule it makes the levain peak about 9% sooner: a modest correction, in the same direction as the second thread.
What the model leaves out
- Fermentation heat. Yeast and bacteria give off heat as they work, and a large mass keeps it. The model has no term for it: it only lets the dough drift toward the room, never away from it. So for a batch big enough to warm itself, the model runs long, and the thread above puts that point at about 4.5–9 kg (10–20 lb) and up, well above a two-loaf bake. A rough heat balance in the model's notes puts the self-heating of a home batch under about 1°C (2°F); no published series of dough temperature against time during bulk was found when the drift model was built, so the size of the effect is not known here.
- Core and surface. The model uses one temperature for the whole batch. In a big tub the center can lag the surface for a while (an estimate from the same notes, not measured), so part of the dough ferments at a different pace than the thermometer shows. Stretch-and-folds mix the two and shrink the difference.
- The container. A wide, shallow tub exposes more surface than a deep bowl of the same dough and follows the room faster. The model's time constant assumes a compact lump.
- The mixer. The friction heat a mixer puts into the dough changes with the batch size, the mixing time and how stiff the dough is, so the starting temperature itself changes with the batch. The DDT formula accounts for that through the friction factor; measure the dough after mixing rather than assuming, and check the mixer's rated capacity before doubling a recipe.
- Scaling the recipe. A triple batch that ferments oddly is sometimes a scaling error: the levain or the salt was not scaled with the flour. Check the percentages before blaming the mass.
What to do with a big batch
- Measure the dough after mixing and enter it in the calculator's dough-temperature field. Start checking at the low end of the calculator's range: the model has no term for the heat a big batch makes, and bakers' reports put that effect at several kilograms and up.
- Mix cooler. The DDT formula gives the water temperature for the dough temperature you want; for a large batch in a stand mixer, raise the friction factor.
- Split the dough into two containers if the tub is deep, so the center is never far from the surface and the folds reach all of it. Each tub then follows the room faster than the calculator assumes for the whole batch, which is another reason to check at the low end of its range.
- Judge by the rise, not the clock: a 50–75% rise in a straight-sided container with a mark, domed and jiggly, is the end of bulk at any size (bulk fermentation by temperature). For a small batch, use a narrow container so the rise is readable.
- Keep the percentages. Levain, salt and water as a percentage of the flour stay the same when you scale; how much starter in sourdough covers the levain.
Frequently asked questions
Does a small batch need more starter?
By the model, no: the same percentage gives the same bulk time at the same temperature. The 25% more mature culture quoted in the thread above is for the levain build of a small home batch, and by the model's starter rule that much more seed makes the levain peak about 9% sooner. If your small batches run slow, a cold bowl or a cooler dough temperature is the more likely reason; measure it.
Does a bigger loaf take longer to proof?
Not in the model's final-proof rule, which depends on the temperature and the bulk, not the loaf size (how long to proof sourdough). A larger shaped loaf does take longer to bake through, which is a different question.
Should I set the calculator for one loaf and multiply?
Set the real number of loaves. The grams scale, and the dough-temperature field uses the whole batch's mass for the drift model, so a six-loaf bake gets the slower drift it should, up to the 3-hour cap.
Plan it with your numbers
Enter the flour per loaf and the number of loaves in the bake schedule calculator, then the dough temperature you measured after mixing. The calculator predicts the bulk over the drift toward your kitchen, with a range for the cooling assumption, and the rest of the schedule follows from it.