Q10 Fermentation Rate Calculator

Fermentation is a chemical process driven by enzymes, and enzyme reaction rates roughly double for every 10C rise in temperature. This relationship is captured by the Q10 coefficient:

rate_new = rate_base x Q10^((T_new - T_base) / 10)

The four figures this page actually uses, and they are the ones in the dropdown:

That 1.8 is worth dwelling on, because it is the one people get wrong.
Vegetable ferments are routinely described as following the same doubling rule as beer, and they do not. Drop a kraut from 21°C to 15°C and the doubling rule says it takes 1.52 times as long; at 1.8 it is 1.42 times. Off a two-week reference that is 21.2 days against 19.9, so a bit over a day, which is the gap between opening the jar on the right day and opening it early.

Practical meaning

If a beer ferments in 7 days at 18C and you raise the temperature to 22C, the rate increases by a factor of 2^(4/10) = 1.32. The beer finishes in about 7 / 1.32 = 5.3 days.

If you drop fermentation temperature from 22C to 10C (to cold-crash or do a slow lager fermentation), the rate drops by 2^(-12/10) = 0.43. A beer that finished in 5 days at 22C would take about 5 / 0.43 = 11.6 days at 10C.

Upper and lower limits

This rule applies within the yeast or bacteria’s active temperature range. Most ale yeasts stop working above 30-35C and go dormant below 10-12C. Lager yeasts function at 8-14C. Sourdough starters stay active down to 4-5C in the fridge but become very slow.

Off-flavor risk. Fermenting too warm does more than just speed things up. It stresses yeast and produces more fusel alcohols and esters. For beer, stay within the yeast manufacturer’s recommended range even if the math says you could go faster.