Sourdough Clock

Sourdough Schedule Calculator: Plan Your Bake Backward from the Oven

By Tam Vu · Updated

On this page
  1. How the calculator works
  2. A worked example
  3. What the model can and can't tell you
  4. Why warmer can taste tangier
  5. Source

Tell the calculator when you want the loaf in the oven and how warm your kitchen is. It works backward and gives you a time for each step: feeding the starter, mixing, shaping and the cold proof in the fridge.

It does not invent how long fermentation takes. You enter how long your own dough and starter took last time at a known temperature, and it scales those times to today's kitchen temperature.

Sourdough schedule

From your last bake

The numbers below are examples, not standards. Replace them with your own.

Works for temperatures from 64 to 86 °F (18 to 30 °C). The fridge step is not scaled; you enter the hours you want. Times are rounded to the nearest 5 minutes.

How the calculator works

The schedule runs backward from the oven:

  1. Shape and chill = oven time − cold proof hours.
  2. Mix = shape time − bulk fermentation hours.
  3. Feed the starter = mix time − hours the starter needs to peak.

The bulk and starter hours come from scaling your reference times. If your dough took 5 hours at 75 °F and today's kitchen is cooler, the calculator multiplies those 5 hours by a time factor greater than 1.

The time factor

Bread yeast and the bacteria in a sourdough starter grow faster as it warms up, up to a point. Gänzle, Ehmann and Hammes (1998) measured this for a yeast and a lactic acid bacterium found in sourdough and fitted a curve to the results. For a temperature T in °C, the relative growth rate is:

γ(T) = a × x^b × e^(−c × x), where x = Tmax − T

For the yeast (Candida milleri), a = 0.029, b = 3.0, c = 0.34 and Tmax = 35.9 °C. The time factor going from your reference temperature to today's is γ(reference) ÷ γ(today).

The table shows the factor relative to 24 °C (75 °F). A factor of 1.63 means the step takes 1.63 times as long.

Kitchen Time factor vs 24 °C Example: 5 h at 24 °C becomes
18 °C (64 °F) 2.26 11.3 h
20 °C (68 °F) 1.63 8.2 h
22 °C (72 °F) 1.24 6.2 h
24 °C (75 °F) 1.00 5.0 h
26 °C (79 °F) 0.88 4.4 h
27 °C (81 °F) 0.86 4.3 h
28 °C (82 °F) 0.88 4.4 h
30 °C (86 °F) 1.07 5.3 h

Notice that the factor stops falling at about 27 °C and then rises. In the study, the yeast grew fastest at 27 °C and stopped growing at 36 °C, so a very warm kitchen can slow the rise.

A worked example

You want the loaf in the oven at 9:00 AM on Saturday. Last time your bulk took 5 hours and your starter peaked 6 hours after a feed, both at 75 °F. Today the kitchen is 68 °F (20 °C) and you will chill the shaped loaf for 12 hours.

  • Time factor from 75 °F (23.9 °C) to 68 °F (20 °C) is about 1.62.
  • Bulk: 5 h × 1.62 = about 8 h 5 min.
  • Starter: 6 h × 1.62 = about 9 h 40 min.
  • Shape and chill: 9:00 AM − 12 h = 9:00 PM Friday.
  • Mix: 9:00 PM − 8 h 5 min = about 12:55 PM Friday.
  • Feed the starter: 12:55 PM − 9 h 40 min = about 3:15 AM Friday.

If a 3:15 AM start does not suit you, move the oven time later or shorten the cold proof, and the whole schedule shifts with it.

What the model can and can't tell you

  • It scales your times, it doesn't predict them. The study measured growth in a lab culture, not in bread dough. That is why the calculator asks for your own last bake as the starting point.
  • Two strains, one curve. The numbers come from specific strains of one yeast and one bacterium. Your starter will be a mix, so treat results as a plan, then watch the dough.
  • Dough temperature, not air. The calculator uses the temperature you enter. If your dough is warmer or cooler than the room, enter the dough temperature.
  • The fridge is outside the model. Cold proof hours are yours to choose. The model covers 18 to 30 °C.
  • Always judge by the dough. Use the schedule to plan, then look at how the dough has risen rather than trusting the clock alone.

Why warmer can taste tangier

The same study found the souring bacterium grows fastest at about 32 to 33 °C, warmer than the yeast. Below about 26 °C the two respond in a similar way. Above that, the bacterium gains ground on the yeast, which fits with warm ferments tasting more sour. The calculator compares the bacterium-to-yeast ratio at today's temperature with the ratio at your reference temperature and reports "tangier", "milder" or "about the same". This is a rough guide from the model, not a pH measurement.

Source