Kombucha second fermentation calculator: sugar per bottle

How much sugar per bottle for a second fermentation?

In the bottle, the yeast in your kombucha turns the added sugar into carbon dioxide and alcohol. Chemistry fixes the most it can make: each gram of glucose or fructose gives at most 0.49 g of CO₂, and table sugar splits into those two sugars first, so a gram of it gives up to 0.51 g. One "volume" of fizz is about 1.96 g of CO₂ per litre, so 500 ml needs about 1 g of CO₂ for every volume you add.

Kombucha straight from its first fermentation is already a little fizzy. The calculator assumes it holds as much CO₂ as 1 atmosphere of the gas allows at your room temperature, about 0.84 volumes at 22 °C (72 °F), and adds only the difference. That is the most an open jar can hold, so the sugar is never overestimated. Juice arrives flat and dilutes that starting fizz a little, which the calculator also counts.

For 2.5 volumes in a 500 ml bottle at 22 °C, every sweetener lands at roughly the same 3.2 to 3.5 g of sugars, because sugar is what the yeast eats:

Sugar figures for honey, maple syrup and juices come from USDA FoodData Central. These amounts assume every gram ferments, which is the ceiling: kombucha's bacteria use some sugar too, and you refrigerate before the yeast has finished, so the real fizz usually lands lower.

How long to second ferment kombucha

The University of California Master Food Preserver Program's kombucha guide (May 2025) says to cap the flavoured kombucha and leave it at room temperature for 1 to 3 days for carbonation, or refrigerate it straight away, and warns that a longer time capped at room temperature can build up enough CO₂ to burst the contents out, especially in warm weather. We have found no tested table of second-fermentation times by temperature, so the calculator's schedule checks a plastic test bottle every 8 or 12 hours and sends everything to the fridge at the three-day mark, firm or not.

Why kombucha bottles burst, and how to stop it

Fizz is pressure. Using the same Henry's-law equation as the carbonation calculator, a bottle at 2.5 volumes and 22 °C pushes about 29 psi (2.0 bar) against its cap, roughly what a soda bottle holds. Fill it with 20 % apple juice, the top of the UC guide's 10 to 20 % range, and if all that sugar fermented it would reach about 5.7 volumes and 86 psi (5.9 bar). That is why the time limit and the fridge matter: you stop the yeast long before it finishes.

• Use bottles made for carbonated drinks. UF/IFAS says appropriately pressure-rated bottles must be used for high carbonation. UC Cooperative Extension Master Food Preservers of El Dorado County suggest thick glass such as heavy swing-top bottles and round rather than square ones, which break less often. Decorative bottles and thin glass are not built for it.
• Add a plastic test bottle. Fill one plastic soda bottle from the same batch. Squeeze it at each check; when it no longer dents, the glass bottles are at the same pressure.
• Refrigerate, then keep it cold. Cold drinks hold their CO₂ at a much lower pressure: the example drops from 29 psi to about 11 psi at 4 °C. The UC guide says both homemade and shop-bought kombucha should be kept in the fridge.
• Go easy on lively flavourings. The El Dorado guide notes that ginger, blueberries and strawberries can increase carbonation, so you may need to "burp" the bottles to let excess CO₂ out.
• Contain it. Keep fermenting bottles together in a closed cupboard or a lidded box, away from where people stand, and open them slowly over the sink.

Kombucha safety basics

The first fermentation is where safety is decided. Colorado State University's food facts say the brew should fall from about pH 5 to 4.2 within seven days and never below 2.5. Guidance for businesses that brew it agrees: Pennsylvania says to discard a batch that has not reached pH 4.2 in seven days, and New York wants it below 4.2 and at least 2.5. A pH meter or narrow-range strips are worth having. Colorado State and the New York guidance say to throw away the culture and the liquid if you see mould, which the El Dorado guide describes as fuzzy blue, grey, green, brown or black patches; brown strings under the culture are yeast and are normal. The CDC advises against brewing or storing kombucha in ceramic or lead crystal containers because of its acidity.

Who should not drink kombucha. The National Capital Poison Center says young children, pregnant women and people with weakened immune systems should not drink it, and Colorado State adds breastfeeding mothers and people with existing health conditions. Start small: the CDC reported that about 4 oz (120 ml) a day may not cause adverse effects in healthy people. If someone is unwell after drinking it, call a doctor or, in the US, Poison Control on 1-800-222-1222. This is general information, not medical advice.

Alcohol in second-fermented kombucha

Every volume of CO₂ that yeast makes leaves behind about 0.26 % alcohol by volume, from the same chemistry. The example adds about 0.43 % ABV if all its sugar ferments, and a soda-like 3 volumes adds about 0.56 %, on top of whatever the first fermentation left. In the US, the Alcohol and Tobacco Tax and Trade Bureau (TTB) treats kombucha that has 0.5 % or more alcohol by volume when bottled, or reaches it later through fermentation in the bottle, as an alcohol beverage under its rules. The El Dorado guide says home kombucha can exceed 0.5 % and in most cases stays under 1 %. Anyone who must avoid alcohol completely should treat home-fermented drinks as containing some.

Common mistakes

• Treating juice as flavour only. Juice is sugar: 20 % juice is several times the sugar a lively fizz needs.
• Forgetting the bottles in a warm room. Warmth speeds the yeast and raises the pressure for the same fizz.
• Bottling before the first fermentation is sour. Leftover sugar from the first fermentation adds to whatever you put in.
• Opening a warm bottle. Chill it first; a cold bottle holds the gas and opens calmly.

For the tea itself, Brew Lyric's tea steeping and caffeine calculators cover steeping times and strength.