The second ferment is where your drink gets its fizz — and where a bottle explodes if you overdo it. Enter your volume, pick a fizz level, and get the exact dose per bottle.
Enter a volume to see the dose.
Keep one plastic test bottle: when it feels hard, carbonation is ready. Move everything to the fridge — cold stops the fermentation.
Valid for a flat drink coming out of an open-vessel first ferment (kombucha, water kefir, ginger beer, kvass, plant soda).
What the maths actually does
The sugar you add at bottling gets eaten by yeast, which breathes out CO₂. Because the bottle is sealed, that CO₂ can't escape: it dissolves into the liquid. That's carbonation.
The chemistry is fixed: one gram of white sugar yields about 0.51 g of CO₂. You need roughly 3.8 g of sugar per litre to add one 'volume' of CO₂ — the unit brewers use. Commercial soda sits around 3 volumes, a fizzy kombucha around 2.
The calculator applies that ratio, then converts for your sugar source: honey is only ~82% fermentable, dextrose is slightly less efficient than white sugar by weight, and fruit juice carries roughly 10 g of sugar per 100 mL.
Why bottles explode
A glass juice bottle isn't built for pressure. Past 8 g of sugar per litre you approach pressures that shatter it, and glass travels. This is the classic beginner accident, and the reason this calculator warns you at that threshold.
Three habits make it a non-issue:
Use pressure-rated bottles: swing-top soda bottles, or champagne bottles.
Keep one plastic test bottle in the same batch. When it feels rock hard, carbonation is ready.
Refrigerate as soon as it's ready. Cold doesn't kill the yeast, it puts it to sleep — pressure stops climbing.
How long does a second ferment take?
At 20-22 °C (68-72 °F), expect 2 to 4 days for medium carbonation. Below 18 °C it can take a week; above 25 °C two days is sometimes enough, and the risk of over-pressure climbs fast.
Time is the wrong measurement — pressure is the right one. That's why the plastic test bottle beats any timer: it tells you what's actually happening in the glass next to it.
Frequently asked questions
How much sugar for a 500 mL bottle of kombucha?
About 2.5 g of white sugar for medium fizz — a little over half a level teaspoon. Plenty of recipes say 'one teaspoon per bottle': that's nearly double, and it's why so many people get a geyser on opening.
Can I use fruit juice instead of sugar?
Yes — it's the most common way to flavour a second ferment. Roughly 10 mL of juice replaces 1 g of white sugar. Careful: sugar content varies enormously between juices (grape ≫ orange), so stay conservative.
What about beer or cider?
This tool deliberately doesn't cover them. Beer at the end of fermentation is already saturated with CO₂ (around 0.85 volumes dissolved); applying this formula would over-carbonate it. Beer priming has to account for residual CO₂ and fermentation temperature.
Do I need sugar if my drink already fizzes during F1?
Yes, if you want fizz in the bottle. In an open jar the CO₂ escapes: the liquid comes out essentially flat, even if it bubbled on the surface. Carbonation is built in a sealed bottle, not before.
Does priming sugar make my drink sweet?
Barely — the yeast eats it. At 5 g/L almost nothing remains on the palate once carbonation is done. If you want a drink that's both sweet and fizzy, you have to halt fermentation in the fridge before all the sugar is consumed.
Let the app handle your second ferments
Inside FermPal this calculator is already filled in with your batch's volume and drink type — right when you move it into the second ferment. No recalculating, no forgotten bottle.