How much sugar should I use per litre of kombucha?

The standard range is 50–80 g per litre of finished tea. Below 50 g/L the SCOBY may stall for lack of fuel; above 80 g/L fermentation slows significantly and the kombucha remains overly sweet. Most home brewers use 60–70 g/L as a balanced starting point.

Why does starter liquid percentage matter?

Starter liquid from a previous batch is acidic (pH 2.5–3.5). Adding at least 10% by volume drops the new batch pH below 4.2, which is the threshold below which harmful bacteria like Aspergillus and Salmonella cannot survive. Using less than 10% is a contamination risk, especially in warmer kitchens.

How is 2F carbonation sugar calculated?

The formula is added_sugar_g = target_CO2_vol × k × batch_litres, where k ≈ 2.7 g/L per volume CO2 for kombucha. This constant is lower than beer (≈ 4 g/L) because kombucha yeast populations are smaller. Any sugar contributed by added fruit juice is subtracted first, so you only add what is genuinely needed.

What is a safe CO2 level for kombucha bottles?

Standard swing-top glass bottles are rated to about 3.0 volumes CO2, which corresponds to roughly 15 psi at room temperature. Most commercial kombuchas sit at 2.0–2.5 vol. Going above 3.0 vol with standard bottles risks breakage. Always use pressure-rated bottles and burp them daily in warm weather.

Can I use fruit juice instead of sugar for 2F?

Yes. Fruit juice contains roughly 100–120 g of sugar per litre depending on the variety. The calculator lets you enter the percentage of juice you are adding and automatically reduces the added cane sugar needed. A 10% juice addition at 110 g/L contribution reduces required added sugar by about 11 g/L.

How long should first ferment take?

At a typical ambient temperature of 22–26 °C, a 1F batch with 60–70 g/L sugar takes 7–14 days. The calculator provides a rough estimate based on sugar concentration. Cooler kitchens slow fermentation significantly — below 18 °C the SCOBY may become dormant. Taste-test daily from day 7 and bottle when the flavour balance suits you.

Kombucha Ratio Calculator

Brewing great kombucha comes down to two sets of numbers: the first ferment ratios that keep your SCOBY healthy and your batch safe, and the second ferment carbonation sugar that produces the fizz without turning your bottles into pressure vessels. This calculator handles both, scaling every figure to your exact batch size.

How it works

First ferment (1F)

The three critical ratios in a 1F batch are sugar concentration, starter liquid percentage, and tea strength. The calculator applies the standard guidelines used by experienced home brewers and backed by fermentation science:

Sugar: 50–80 g per litre of finished tea. The SCOBY bacteria (primarily Acetobacter and Gluconobacter) and the wild yeasts convert sucrose to acetic and gluconic acid plus CO₂. Too little sugar and fermentation stalls; too much and the batch stays unpleasantly sweet.
Starter liquid: at least 10% of total volume. Acidic starter liquid (pH 2.5–3.5) drops the new batch below pH 4.2, above which dangerous contaminants can take hold.
Tea bags: roughly one standard bag per 500 ml of water is a practical baseline for medium-strength black or green tea; adjust to taste.

The calculator also estimates residual sugar after your planned ferment duration using an empirical model: approximately 30% of initial sugar is consumed by day 7, rising to around 60% by day 14 at 22–24 °C. This gives you a rough idea of how sweet the finished 1F liquid will be before you move to bottles.

Second ferment (2F)

Carbonation in a sealed bottle comes from yeast consuming additional sugar and producing CO₂ that cannot escape. The formula is:

added_sugar (g) = target_CO₂ (vol) × k × batch_litres

where k ≈ 2.7 g/L per volume CO₂ for kombucha. This constant is deliberately lower than the standard beer priming figure of ~4 g/L because kombucha yeast populations are smaller and less predictable. If you add fruit juice, its sugar content (~110 g/L for typical fruit juice) is subtracted before the added cane sugar is calculated.

Worked example

1F batch — 3 litres:

You are brewing a 3-litre batch with 65 g/L sugar and 15% starter liquid:

Ingredient	Amount
Sugar	195 g
Starter liquid	450 ml
Tea / water	2,550 ml
Tea bags (approx)	5

The calculator estimates residual sugar after 10 days at about 98 g — enough sweetness to feed the 2F yeast without the drink being cloying.

2F carbonation — 2.5 litres into 500 ml bottles with 10% apple juice:

Parameter	Value
Target CO₂	2.5 vol
Juice sugar contribution	~27.5 g
Added cane sugar	~40 g
Per bottle (500 ml)	~8 g
Estimated pressure	~11 psi

At 2.5 vol CO₂ and 11 psi the batch is well within the safe range for swing-top glass bottles rated to ~3 vol / 15 psi.

Formula note

The carbonation sugar formula uses the empirical kombucha priming constant k = 2.7 g/L/vol, derived from the lower yeast activity in live kombucha compared to beer. For a target of T volumes CO₂ in a batch of V litres, the total added sugar S is:

S = T × 2.7 × V (grams)

Divide S by the number of bottles to get the per-bottle dose. The pressure estimate uses a simplified linear interpolation across measured reference points (1.5–3.5 vol at ~20 °C) for standard glass swing-top bottles.