What each of the four numbers actually controls
Kombucha is a two-organism ferment. Yeasts convert sucrose into ethanol and carbon dioxide; acetic acid bacteria convert that ethanol into acetic acid, with gluconic and glucuronic acids alongside it. Every number in the recipe feeds one side of that partnership.
Sugar is the yeast's entire food supply, and through them the bacteria's. Too little and the culture weakens across successive batches; too much and the ferment takes longer to reach the acidity you want, finishing sweet. The usual working band is 50–70 g/L. The traditional "one cup of sugar per gallon" sits at the bottom of it: a US cup of granulated sugar weighs about 200 g, which over 3.785 L is 53 g/L.
Tea supplies nitrogen compounds, purines and polyphenols that the culture needs, and it is not optional — a sugar-and-water ferment produces a weak, thin culture within a few generations. Around 5–8 g/L of loose leaf is normal, which at roughly 2 g per tea bag is two to four bags per litre. Camellia sinensis of any colour works; herbal infusions do not, because they lack the compounds the culture depends on.
Starter liquid is the safety control. It is mature, acidic kombucha from a previous batch, and pouring 10–20% of the batch volume in drops the pH immediately rather than waiting days for the culture to produce acid. That head start is what keeps surface moulds out, and it does far more work than the pellicle floating on top.
Priming sugar is the only number that has nothing to do with fermentation safety. It is fuel for a sealed second ferment, where yeast converts it into dissolved CO₂ and the drink becomes fizzy.
Why the sugar is dosed on the finished volume, not the water you boil
The arithmetic is deliberately simple: multiply the batch volume by each rate. What matters is which volume you multiply.
Dose on the finished batch volume — everything that will be in the vessel, starter included. The starter liquid is old kombucha, so its residual sugar is close to zero and its tea has already been consumed. If you brewed sweet tea at 60 g/L and then added 15% starter on top, the finished batch would sit at 51 g/L, noticeably weaker than you intended, and it would drift weaker every time you scaled the recipe up.
Doing it the other way round is one subtraction. Multiply the full batch volume by the sugar rate, then brew that sugar into only the sweet-tea portion. For a gallon at 60 g/L with 15% starter: 227 g of sugar goes into 3.22 L of tea, which is 71 g/L in the pot and exactly 60 g/L once the starter joins it.
Bottle counting uses a floor, not a round. Five and a third bottles is five bottles and a glass to drink now, because a partly filled bottle carbonates against a large headspace and stays flat. The priming total follows from the bottle count, so it updates with your bottle size.
One thing the calculator deliberately does not model is the fermentation itself. How long the primary ferment takes depends on temperature, culture vigour and how sour you like it — seven to fourteen days at 24–27 °C is typical. Taste with a straw from day five and bottle when it suits you.
Worked example: scaling a one-gallon recipe to two gallons
Your house recipe is one US gallon at 60 g/L sugar, 6 g/L tea and 15% starter, bottled into 500 mL swing-tops with 5 g of sugar each. You have bought a two-gallon vessel.
- Convert the target volume. 2 US gal × 3.785412 = 7.5708 L.
- Sugar. 7.5708 × 60 = 454 g. That is 2¼ cups at 200 g per cup — and note it is exactly double the gallon figure of 227 g, which is the point of working in ratios.
- Tea. 7.5708 × 6 = 45.4 g of loose leaf, or about 23 standard bags.
- Starter. 7.5708 × 15 ÷ 100 = 1.136 L of mature kombucha. If your last batch cannot spare that much, either reduce the batch size or make up the difference with distilled white vinegar, never with plain water.
- Sweet tea to brew. 7.5708 − 1.136 = 6.435 L. Brew the 45.4 g of tea and dissolve all 454 g of sugar into that 6.435 L, then cool it before the starter goes in.
- Bottles. 7,570.8 mL ÷ 500 = 15.14, so 15 bottles and a little left over. Priming: 15 × 5 = 75 g of sugar in total.
The check that catches scaling errors: divide the sugar by the finished volume. 454 ÷ 7.5708 = 60 g/L. If that number does not come back to your target, you dosed on the wrong volume.
Reading the result and knowing when the batch is right
The number that decides safety is not on this page — it is pH. A healthy kombucha reaches a pH of 4.2 or below within a couple of days of pitching, and finishes somewhere between 2.5 and 3.5. Cheap pH strips are enough to confirm the first drop; if a batch has not acidified within 72 hours, the starter share was too small or the culture is exhausted, and the right move is to discard it rather than wait.
Sweetness is the number you taste for. Kombucha that still tastes like sweet tea after two weeks is either cold or under-pitched; kombucha that tastes like vinegar has gone past the point most people enjoy, though it makes excellent starter for the next batch and a decent salad dressing.
For the second ferment, 5 g of sugar in a 500 mL bottle is about 10 g/L, which is a firm carbonation over three to five days at room temperature. Fruit juice counts towards that figure — 50 mL of apple juice carries roughly 5 g of sugar — so if you flavour with juice, drop the added sugar accordingly. Bottles left too long or kept too warm build genuinely dangerous pressure, and glass that is not pressure rated can fail. Refrigerate to stop the ferment, and open cold and over a sink.
Alcohol is worth a word. Kombucha always contains some ethanol as an intermediate, usually well under 1% ABV in a fully fermented batch, but a sealed second ferment with plenty of sugar and a yeast-heavy culture can go higher. If that matters to you, ferment the second stage briefly and cold.
Standard batch sizes at common concentrations
| Batch | Sugar at 50 g/L | Sugar at 60 g/L | Sugar at 70 g/L | Tea at 6 g/L | Starter at 15% |
|---|---|---|---|---|---|
| 1 US qt (0.946 L) | 47 g | 57 g | 66 g | 5.7 g | 142 mL |
| 1 L | 50 g | 60 g | 70 g | 6.0 g | 150 mL |
| 1 US gal (3.785 L) | 189 g | 227 g | 265 g | 22.7 g | 568 mL |
| 2 US gal (7.571 L) | 379 g | 454 g | 530 g | 45.4 g | 1.14 L |
| 5 US gal (18.93 L) | 946 g | 1,136 g | 1,325 g | 113.6 g | 2.84 L |
One US cup of granulated sugar weighs about 200 g, so the 1 gallon at 50 g/L row is close to the traditional one-cup-per-gallon recipe.
Mistakes that cost you a batch
- Topping up with water instead of starter. Diluting a batch with plain water raises the pH and removes the acid protection. If you are short of starter, use distilled white vinegar to make up the difference.
- Adding starter to hot tea. Cool the sweet tea to room temperature first. Sustained contact much above 40 °C damages the yeasts and bacteria you are trying to keep alive, and hot tea poured onto a pellicle kills it outright.
- Using flavoured or herbal tea for the primary ferment. Oils in bergamot and many flavoured blends inhibit the culture, and herbal infusions lack the nitrogen source it needs. Flavour at the second ferment instead.
- Sealing the primary ferment. The first stage needs air for the acetic acid bacteria. Cover with tightly woven cloth; a sealed lid belongs only on the second ferment.
- Fermenting in the wrong vessel. Glass, food-grade plastic or stainless steel only. Decorative ceramic glazes and unlined metals can leach into an acidic liquid.
- Over-priming or over-warming the second ferment. Sugar in a sealed pressure-rated bottle is how you get carbonation; sugar in a sealed jam jar left on a warm windowsill is how you get glass across the kitchen.
Key terms
- Starter tea
- Mature, unflavoured kombucha from a previous batch, added to acidify a new one immediately. It is the primary defence against mould, and it matters more than the pellicle.
- Pellicle (SCOBY)
- The cellulose mat produced by the bacteria at the liquid surface. It is a by-product and a partial barrier, not the culture itself — the organisms live throughout the liquid.
- Primary ferment
- The open, cloth-covered stage in which sugar is converted to acids over one to two weeks. Requires air.
- Second ferment
- The sealed, bottled stage in which a small amount of added sugar is converted to dissolved carbon dioxide over a few days at room temperature.
- Continuous brew
- A vessel with a tap from which you draw finished kombucha and to which you add fresh sweet tea, keeping a large acidic reservoir permanently in place. The same ratios apply to whatever fraction you replace.
How kombucha compares with the other ferments in this category
Every ferment on this site is a controlled competition, and it is instructive to see which control each one uses.
Kombucha uses acid, delivered up front. The starter liquid puts the vessel below pH 4.2 on day one, before anything unwanted can establish. A vegetable ferment uses salt to hold the line for the first few days until lactic acid bacteria produce their own acid — a slower handover, which is why salt percentage matters so much there. Cured meat uses salt plus sodium nitrite at a regulated ingoing concentration, because the conditions inside a sausage are anaerobic and neither acid nor salt alone controls botulinum reliably.
The mechanical parallel is with brewing rather than preserving. Priming sugar in a kombucha bottle does exactly what priming sugar does in a bottle-conditioned beer: a measured dose of fermentable sugar in a sealed pressure-rated vessel, converted to dissolved CO₂. The dose per litre is similar, and so are the consequences of getting it wrong.
What kombucha never involves is heat preservation. Bottled kombucha is a live, refrigerated product; putting it through a boiling-water bath with an altitude-corrected process would kill the culture, drive off the carbonation and leave you with sour tea. If you want a shelf-stable drink, that is a different product entirely. And the percentage discipline shared with dry brining and sausage making is the transferable skill: write the recipe as rates, and it scales to any vessel you own.