Kombucha Batch Builder Calculator: F1, F2 and Carbonation Guide

What Every New US Kombucha Brewer Needs to Know Before Brewing

A first-time kombucha brewer in Austin buys a SCOBY from her local homebrew shop and brews her first one-gallon batch. She steeps the tea and adds sugar correctly, but she only uses 5 percent starter tea instead of the recommended 10 percent because she does not want to waste her precious first starter. Three days later, she notices an orange fuzzy growth on the surface. That is mold, not a new SCOBY layer. The batch is ruined. The starter tea was insufficient to acidify the sweet tea quickly enough, and harmful mold spores won the race. She has to throw out the whole batch and start over. If she had used the correct 10 percent starter ratio, the batch would have been fine.

Kombucha is one of the most accessible fermented beverages to brew at home, but it has exactly two non-negotiable safety rules: enough starter tea (minimum 10 percent of batch volume, pH below 3.5) and never use metal containers in contact with the kombucha (the acids corrode metal and can leach harmful compounds). Everything else, including tea type, sugar amount, fermentation time, and flavor additions, is flexible and forgiving. This calculator handles the math for both F1 (first fermentation, the main brew) and F2 (second fermentation, the carbonation stage) so you can focus on the craft.

Understanding the SCOBY and Its Needs

The SCOBY is a living culture that thrives in specific conditions. It needs: the tannins and trace nutrients found in real Camellia sinensis tea leaves (not herbal teas alone), a specific sugar concentration to ferment, temperatures between 68 and 86 degrees Fahrenheit, and acidic pH maintained by starter tea. A healthy SCOBY is creamy to tan or light brown in color, firm and rubbery in texture, and at least one quarter inch thick. It will produce a new SCOBY layer (called a “baby”) on top of the liquid surface with each batch. Both the mother and baby SCOBYs can be used for future batches. The baby is typically removed and either used, shared, or stored in a SCOBY hotel.

How This Kombucha Batch Calculator Works: F1, F2 and Bottle Ratios

F1 Calculator: Ingredient Amounts and Fermentation Time

Enter your batch volume in gallons, tea type, sweetness preference, starter tea percentage, and fermentation temperature. The calculator converts gallons to quarts, separates the batch into starter tea volume and water to boil (total minus starter), then computes sugar grams (quarts of water times sweetness factor), tea bags (quarts times bags per quart for the tea type), and a fermentation time estimate based on temperature. The step-by-step brew sheet in the output tells you exactly what to do in order, including the correct steeping temperature and time for your tea type.

F2 Calculator: Per-Bottle Carbonation

Enter total kombucha volume in ounces (1 gallon equals 128 oz), bottle size, desired carbonation level, sugar source, and F2 temperature. The calculator shows per-bottle and total amounts for your selected source, the total bottle count, and estimated F2 time. The comparison chart shows all five source options side-by-side for your bottle size and carbonation level, helping you decide between plain sugar, honey, fruit juice, or fresh fruit based on what you have available.

pH Guide and Safety

pH is the key safety check in kombucha brewing. The output card always shows the target pH range (2.8 to 3.5) alongside the fermentation estimate. Never bottle kombucha above pH 3.5, as the acidity may not be sufficient to prevent pathogen growth in sealed bottles at room temperature. A calibrated digital pH meter is strongly recommended for home brewing. Test strips can give a rough reading but are not accurate enough at the low pH values of finished kombucha for safety purposes.

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Expert Tips for Healthy SCOBY and Perfect Kombucha Carbonation at Home

Test pH Every Single Batch Before Bottling

The single most important habit for safe and consistent kombucha brewing is testing pH before transferring to sealed bottles. A $15 to $30 digital pH meter (available on Amazon from brands like Apera or Vivosun) gives you an accurate reading in seconds. Insert the clean probe into the kombucha after removing the SCOBY, wait for a stable reading, and compare to the 2.8 to 3.5 target range. If pH is above 3.5, leave the batch to ferment another 2 to 4 days and retest. Never skip this step when first brewing. After you have brewed dozens of consistent batches and know exactly how your kombucha behaves, you can start trusting taste alone, but not before.

The Squeeze Test for F2 Carbonation

If you use plastic PET bottles for F2 (BPA-free, sold specifically for kombucha), the squeeze test is your best friend: when the bottle goes from pliable to firm, the CO2 from F2 fermentation has built up and your kombucha is carbonated. Squeeze a test bottle every 12 to 24 hours during F2. The moment it feels firm (comparable to a carbonated soft drink bottle), put all bottles in the refrigerator immediately. The cold stops yeast activity and locks in the carbonation. If you use glass swing-top bottles, open one gently over a sink daily to check carbonation; this also releases any excess pressure to prevent overpressure buildup.

Keep the SCOBY Submerged During F1

During F1, the SCOBY may float on the surface, sink to the bottom, stand vertically, or do any combination of these. None of these positions are wrong. A SCOBY at the bottom is equally effective as one floating at the top. What matters is that the SCOBY stays below the liquid surface and does not sit exposed to air above the kombucha level, where it can dry out and mold may form. If the SCOBY is floating partially out of the liquid, use a clean hand to push it gently back under the liquid. The new SCOBY layer always forms at the liquid surface regardless of where the old one is.

Quick Reference: Kombucha Brewing Ratios and F2 Carbonation Sugar Guide

F1 ratios per quart of water. F2 amounts per 16oz bottle at medium carbonation. Tea steep time and temperature from tea manufacturer best practices. pH target and safety guidance from the Kombucha Brewers International (kombuchabrewers.org).

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