Kombucha Brewing Calculator

The Art and Science of Kombucha Brewing

Kombucha is a fermented tea drink that's been brewed for over 2,000 years. A SCOBY (symbiotic culture of bacteria and yeast) transforms sweet tea into a tangy, naturally carbonated probiotic beverage. The fermentation process consumes most of the sugar, converting it into organic acids, CO2, and trace amounts of alcohol (typically 0.5-1% ABV).

Getting the ratios right is critical. Too little sugar starves the SCOBY, producing weak, flavorless kombucha. Too much sugar overwhelms fermentation and results in an overly sweet product. The ratios in this calculator represent the accepted standard among experienced brewers, but you can adjust sugar levels to taste once you're comfortable with the process.

If you're also interested in fermented foods, our sourdough hydration calculator follows similar ratio-based principles for bread baking, and the homebrew ABV calculator can help track alcohol content in more potent fermented beverages.

Kombucha Ratio Formula

Per gallon: 1 cup sugar + 4-6 tea bags + 2 cups starter tea
Water = Total volume − starter tea
Yield = Total volume − SCOBY reserve (~4 cups)

Why These Ratios Work

The 1 cup per gallon sugar ratio creates a 5-6% sugar solution — the optimal concentration for SCOBY metabolism. The SCOBY feeds on sucrose, breaking it into glucose and fructose, then converting those into acetic acid, gluconic acid, and other beneficial organic acids that give kombucha its characteristic tang.

Starter tea serves dual purposes: it acidifies the brew to pH 4.0-4.5 immediately, inhibiting mold and harmful bacteria, and it introduces active bacteria and yeast populations that jump-start fermentation. Skipping or reducing starter tea is the most common cause of contaminated batches.

Step-by-Step Kombucha Brewing Process

1. Boil water and steep tea. Bring half your water to a boil, remove from heat, and steep tea bags for 5-10 minutes. Don't over-steep or the kombucha will taste bitter.
2. Dissolve sugar. While tea is hot, add sugar and stir until completely dissolved. This is critical — undissolved sugar can create hot spots that stress the SCOBY.
3. Cool the sweet tea. Add remaining water (cold) to cool the mixture to room temperature (below 85°F). Never add the SCOBY to hot liquid — it will kill the culture.
4. Add starter tea and SCOBY. Pour cooled sweet tea into a clean glass jar, add starter tea, then gently place the SCOBY on top. It may sink — that's normal.
5. Cover and ferment. Cover with a tightly woven cloth or coffee filter secured with a rubber band. This allows airflow while blocking fruit flies. Place in a warm (72-80°F), dark spot away from direct sunlight.
6. Taste test starting day 5. Use a clean straw to sample daily. When the balance of sweet and tart suits your taste, fermentation is complete.
7. Bottle and second ferment (optional). Reserve 2 cups of kombucha and the SCOBY for your next batch. Pour the rest into swing-top bottles with fruit or juice for 2-4 days of secondary fermentation to build carbonation.

Common Kombucha Brewing Mistakes

• Using the wrong container. Always brew in glass, food-grade ceramic, or stainless steel. Never use plastic (chemicals leach into acidic kombucha), crystal (contains lead), or reactive metals (copper, aluminum). Wide-mouth jars work best for oxygen exchange.
• Insufficient starter tea. Using less than 10% starter tea by volume is the top cause of mold contamination. When in doubt, add more. Extra acidity from starter tea never hurts the brew.
• Brewing too cold. Below 65°F, fermentation slows dramatically and mold risk increases. If your house is cool, use a seedling heat mat or kombucha heating wrap to maintain 72-80°F.
• Bottling too early or too late. Too early = too sweet, not enough carbonation in F2. Too late = too vinegary, less pleasant to drink. Taste daily and bottle when it's slightly sweeter than you want — F2 consumes more sugar.
• Not burping bottles during F2. Pressure builds fast during second fermentation, especially with fruit additions. Open (burp) bottles once daily to release excess pressure. Use pressure-rated bottles, and never use regular glass jars.

Worked Example: 2-Gallon Batch with Green Tea

Brewing a 2-gallon batch using green tea as the base:

• Water: 2 gallons (32 cups) minus 4 cups starter = 28 cups water
• Sugar: 2 cups (400g) cane sugar
• Tea: 10 green tea bags (or 8 tsp loose-leaf)
• Starter tea: 4 cups from previous batch
• Expected yield: ~28 cups = fourteen 16-oz bottles

Green tea kombucha ferments slightly faster than black tea (5-10 days vs 7-14 days) and produces a lighter, more delicate flavor. For second fermentation, green tea kombucha pairs well with ginger, lemon, or berries.