Fermentation Calculator

Fermentation time and temperature are the two most critical variables in any fermentation project. Temperature governs how fast the microbes work.
Too cold and fermentation stalls. Too hot and you either kill the culture outright or push it into producing off-flavors, which is the more common way it goes wrong.

General fermentation rate relationship: Estimated Days = Reference Days × Q10^((T_reference − T_actual) / 10)

Q10 is how many times faster the culture works when you give it another 10°C. For yeast it is 2, so a beer that takes 7 days at 20°C takes roughly 3.5 days at 30°C, and 14 days at 10°C.

Not every culture uses 2, and the difference is bigger than it looks. This page uses 2.5 for a sourdough starter and 1.8 for the Lactobacillus in sauerkraut and kimchi, matching the figures on the Q10 fermentation rate calculator.
A sourdough is a mixed culture and responds harder to a cool kitchen than a straight yeasted dough. A kraut responds less. Run a kraut from 21°C down to 15°C on a flat Q10 of 2 and you overshoot the time by about 7%, which is a day and a bit on a fortnight.

The rule holds only inside the range the organism can live in. Push past that and it stops being a rate law and starts being a death certificate: brewing and bread yeasts give up somewhere around 40°C, the Lactobacillus in a vegetable ferment holds out to about 45°C, and a yogurt culture, which is thermophilic to begin with, to 50°C. Above those points a faster answer is not a real answer, so this calculator refuses to give one.

Reference points used by this calculator. Each row gives a temperature and the time you should expect at that temperature. The Q10 rule above then scales from there for whatever your kitchen is actually doing.

Sauerkraut and kimchi share a temperature but not a timeline, which surprises people who lump them together. Kimchi is usually eaten young and then moved to the fridge to carry on slowly; sauerkraut is normally left to go properly sour, and two weeks at room temperature is a fair starting point. Both are ready when they taste ready, so start sampling well before the number above.

This page answers “how long”, and the temperature is the input. The fermentation temperature calculator answers “is this temperature right”, and the time comes out as a by-product.
They run the same Q10 rule off the same reference points, the same death and stall temperatures, and the same practical caps, so a 30°C ale reads 5 days on both. Use whichever question you actually have. That page also covers kombucha and tempeh, which are not in the table above.

Salt concentration for vegetable ferments: Salt % by weight = Salt / (Vegetables + Salt) × 100 Ideal range: 2–3% salt by weight, which inhibits harmful bacteria while still letting Lactobacillus thrive.

Worked example, sauerkraut: 1 kg cabbage | 20 g salt: Salt % = 20 / (1000 + 20) = 1.96% → slightly low; use 25 g for a safer 2.4%.

pH monitoring: Successful vegetable fermentation drops pH from about 6 to below 4.6 within the first 48 hours. Under that level, harmful bacteria cannot survive, which is the whole point of the salt.