For a same-day sourdough, set the proofing box to about 24–26°C (75–79°F). By the calculator's model that brings bulk with 20% levain to about 5 h to 4 h, compared with about 7½ h in a 20°C (68°F) kitchen: about 33% shorter at 24°C (75°F), 47% shorter at 26°C (79°F) and 57% shorter at 28°C (82°F). The question "what temperature should I set" has two halves, how fast you want the dough to move and what you want it to taste like, and the model answers the first half. This guide gives the model's bulk, levain and final-proof times for each setting up to 30°C (86°F), the top of the bake schedule calculator's range, explains how to get the dough to the box's temperature as well as the air, and shows where the flavor question comes in. It recommends no particular box; a warm oven or a heat mat does the same job.
What each setting does, by the model
The table is the model's prediction when the dough sits at the temperature in the first column for the whole stage. The 20°C and 22°C (68°F and 72°F) rows are ordinary kitchens, for comparison. Bulk ends at a 50–75% rise; the levain is built 1 part starter to 2 parts flour and 2 parts water; the final proof is the calculator's rule of thumb for a proof after a full bulk, about half the bulk time (how long to proof sourdough).
| Box or room temperature | Bulk with 20% levain | Bulk with 10% levain | Levain 1:2:2 to its peak | Final proof, typical |
|---|---|---|---|---|
| 20°C (68°F) | about 7½ h | about 9¾ h | about 6½ h | about 3¾ h |
| 22°C (72°F) | about 6 h | about 8 h | about 5½ h | about 3 h |
| 24°C (75°F) | about 5 h | about 6½ h | about 4½ h | about 2½ h |
| 26°C (79°F) | about 4 h | about 5½ h | about 4 h | about 2 h |
| 28°C (82°F) | about 3¼ h | about 4½ h | about 3¼ h | about 1¾ h |
| 30°C (86°F) | about 2¾ h | about 3¾ h | about 2¾ h | about 1½ h |
The model behind the bulk columns is the one in bulk fermentation by temperature: about 2.2 times faster for every 8°C (14°F) warmer, and about a third longer with half the levain; the levain column follows the starter model, about twice as fast per 8°C. Rows above 23°C (73°F) are warmer than the test doughs the levain term was fitted to, so they are the model's extrapolation. The table stops at 30°C (86°F), the end of the calculator's slider; the calculator does not plan above it, and summer sourdough shows 32°C (90°F) as an extrapolation. The levain peaks sooner in the box too: about 4 h for a 1:2:2 feed at 26°C (79°F) compared with about 5½ h at 22°C (72°F), and a levain that peaks sooner also falls sooner, so look at it rather than the clock (sourdough starter not doubling has the peak times for other feeds).
Picking a proofing box temperature
- 24–26°C (75–79°F) is the setting the calculator's own tip suggests for a cold kitchen (it uses 26°C): a bulk of about 4–5 h with 20% levain, fast enough for a same-day loaf and slow enough to fit the stretch-and-folds in the first two hours and still see the dough change. It is also the range the warm spots in winter sourdough aim for.
- 28–30°C (82–86°F) moves bulk to about 3¼ h and 2¾ h. That is fast: the rise you are watching for arrives little more than an hour after the last fold, and a missed check costs more. The model's temperature rule was not checked above 30°C, and yeast activity flattens as it nears its optimum, so expect less gain at the top end than the rule says. If you want the warmth but a slower clock, cut the levain: at 26°C (79°F) the model predicts about 5½ h of bulk with 10% levain compared with 4 h with 20%.
- Flavor. The model predicts the rise and says nothing about taste. It is commonly said that warm, short fermentations give a milder, more lactic loaf and cool, long ones a sharper, more acetic one, but the published evidence for that is thin, and warmer dough makes acid faster. Treat the setting as a timing choice first; a cold retard changes the flavor more, as common practice has it, and a loaf at each end of the range tells you what your starter does.
Whatever the setting, measure it. The dial on a box, and the light in an oven, say less than a thermometer placed next to the dough, and a box heated from below can run warmer at the bottom than at the top.
Get the dough to the box's temperature, not just the air
The box warms the air; the dough warms slowly. A dough that is at 22°C (72°F) after mixing and goes into a 28°C (82°F) box does not ferment at 28°C from the start: the model has it drifting toward the box over the first hours and predicts about 4¼ h of bulk for a 1 kg batch (up to 4½ h if it warms slowly) compared with 3¼ h for dough that is at 28°C all along. So mix the dough to about the box temperature. Water at the box temperature is not enough on its own, because the flour and the bowl are at kitchen temperature: the DDT formula gives the water temperature for a target dough temperature, and in a 20°C (68°F) kitchen it comes out at roughly 35–40°C (95–104°F) water for a 27°C (81°F) dough by hand (keep the water at or below about 40°C as common practice). Then measure the dough. Does batch size change bulk time explains the drift model and why a big batch takes longer to warm through.
In the calculator, set Room temperature to the box temperature and it recalculates the schedule for the box. If the dough goes in cooler than the box, enter the measured dough temperature in the optional field and the calculator accounts for the warm-up in the bulk time.
Without a box
- The oven with the light on. An incandescent oven light warms many ovens to about 24–26°C (75–79°F) with the door closed; LEDs barely warm anything, some ovens climb past 30°C (86°F), and some newer ovens switch the light off after a few minutes, so leave a thermometer in for an hour first. Aim for 24–26°C (75–79°F) and prop the door open with a wooden spoon above about 27°C (81°F), as the calculator's tip says. Tape a note to the oven controls so nobody preheats the oven with the dough inside, and take the dough, and any plastic tub, out before the oven is used for anything else.
- A heat mat and a tub. A seed-starting mat under a lidded plastic tub warms the tub several degrees above the room; how much depends on the mat and the kitchen, and a mat without a thermostat is not steady. With a thermostat controller set to about 26°C (79°F) and its probe inside the tub it holds the temperature; without one, measure inside the tub and put a folded towel or a rack between mat and tub if the bottom runs hot.
- Warm spots. The top of the fridge, a cabinet near a heat vent or radiator, an insulated cooler with a jar of warm water. All of them need a thermometer before you trust them; winter sourdough compares them.
A box, bought or improvised, only helps if the dough is in it for the whole stage. Taking the tub out for folds every half hour in a cold kitchen cools it each time; fold quickly and put it back.
The final proof in the box
After shaping, the model predicts a final proof of about 2 h at 26°C (79°F) with 20% levain, starting the poke test at about 1½ h. A warm final proof is quick and the window is short, so many bakers proof the shaped loaf at room temperature or retard it in the fridge instead; a cold loaf is also easier to score, and cold retard vs. same-day weighs the two. If the loaf proofs in a separate box or on a mat, start a 45- to 60-minute preheat (Dutch oven vs. baking stone) about half an hour after the loaf goes in, so the oven is hot by the first poke test. If your box is the oven, take the loaf and any plastic out first and let it finish, covered, at the warmest spot in the kitchen while the oven heats.
Frequently asked questions
Is 30°C (86°F) too hot for sourdough?
Not for the dough; it is the fastest setting the calculator plans for, with a bulk of about 2¾ h at 20% levain, and the model is not built for anything warmer. The practical problem is the pace: by the model the final-proof window at 30°C runs from about 1 h to 2 h, so stay close and check often, or use less levain.
What about the yogurt setting, 35–40°C (95–104°F)?
Outside the calculator's range: the model is not built for it and gives no time. It is also past the point where the yeasts in a sourdough stop growing while the bacteria keep making acid, so the rise slows as the dough sours. Warm the dough to 24–28°C (75–82°F) instead, and if your box only has a yogurt setting, use it to prewarm the box and switch it off, with a thermometer inside.
Should the levain go in the box too?
Yes, if it fits. The levain peaks on its own clock, in the table's fourth column, and the dough's bulk starts when the levain is ripe. With the jar and the tub at the same temperature the two stay in step, which is what the calculator's one-temperature plan assumes.
Plan it with your numbers
Set Room temperature in the bake schedule calculator to the box temperature, enter your levain amount, and read the levain, bulk and proof times for that setting. If you will mix cooler than the box, enter the dough temperature you expect in the optional field and the calculator adjusts the bulk for the warm-up.