Fermentation Time Calculator Formula

Understand the math behind the fermentation time calculator. Each variable explained with a worked example.

Use the Fermentation Time Calculator

Formulas Used

Minimum Fermentation Time

min_days = round(base_days * temp_factor * salt_factor * 0.7 * 10) / 10

Ideal Fermentation Time

ideal_days = round(base_days * temp_factor * salt_factor * 10) / 10

Maximum Fermentation Time

max_days = round(base_days * temp_factor * salt_factor * 1.5 * 10) / 10

Variables

VariableDescriptionDefault
ferment_typeFermentation Type1
temp_fAmbient Temperature(°F)70
salt_pctSalt % (weight of salt / weight of vegetables)(%)2.5
base_daysDerived value= (ferment_type == 1) * 21 + (ferment_type == 2) * 10 + (ferment_type == 3) * 0.5 + (ferment_type == 4) * 7calculated
temp_factorDerived value= 70 / temp_fcalculated
salt_factorDerived value= 1 + (salt_pct - 2.5) * 0.2calculated

How It Works

How Fermentation Time Is Estimated

Key Factors

  • Temperature: Warmer environments speed up fermentation. 65-75°F is ideal for most lacto-fermentation.
  • Salt: Higher salt slows fermentation. Standard sauerkraut uses 2-3% salt by weight.
  • Ferment type: Each type has a different base timeline.

    • Base Times (at 70°F, 2.5% salt)

  • Sauerkraut/Kimchi: 14-28 days
  • Kombucha: 7-14 days
  • Yogurt: 8-12 hours
  • Sourdough starter: 5-10 days to establish

    • Worked Example

    Estimate sauerkraut fermentation time at 70°F with 2.5% salt.

    ferment_type = 1temp_f = 70salt_pct = 2.5
    1. 01Base time for sauerkraut = 21 days.
    2. 02Temp factor = 70 / 70 = 1.
    3. 03Salt factor = 1 + (2.5 - 2.5) x 0.2 = 1.
    4. 04Ideal time = 21 x 1 x 1 = 21 days.
    5. 05Minimum = 21 x 0.7 = 14.7 days.
    6. 06Maximum = 21 x 1.5 = 31.5 days.

    Frequently Asked Questions

    How do I know when fermentation is done?

    Taste it regularly. Sauerkraut and kimchi are done when they reach your preferred level of sourness. Kombucha should be tangy and lightly fizzy. There is no exact endpoint; personal preference determines doneness.

    Is it safe to ferment at home?

    Yes. Lacto-fermentation is one of the safest food preservation methods. The acidic, salty environment prevents harmful bacteria. Keep vegetables submerged below the brine, use clean equipment, and trust your senses.

    What if white film appears on top?

    A thin white film (kahm yeast) is harmless but can add off-flavors. Skim it off and continue fermenting. Fuzzy mold (pink, green, or black) means the batch should be discarded.

    Ready to run the numbers?

    Open Fermentation Time Calculator