Short answer
Regular shop kombucha is generally halal. Its small trace of alcohol forms naturally in fermentation, and drinks sold as non-alcoholic kombucha in the US must stay under 0.5 percent, an amount that cannot intoxicate however much you drink. Hard kombucha, brewed as an alcoholic drink, is haram.
A fermented drink is halal as long as it has not become intoxicating. Kombucha with a natural trace of alcohol that cannot intoxicate in any quantity is permissible; hard kombucha, which intoxicates, is haram in any amount.
The four schools agree on this.
Explanation
Kombucha is sweet tea fermented with a culture of yeast and bacteria. Like all fermentation, it makes a little alcohol, and the bacteria turn much of that into acid. The four schools agree on the test: a drink becomes forbidden when it becomes intoxicating, and a drink that cannot intoxicate is not khamr.
The Prophet ﷺ drank nabidh, water in which dates or raisins had been soaked, for up to three days, and then had it thrown away before it could turn. This shows that a drink which has begun to ferment is not haram by that alone; the line is crossed when it can intoxicate.
- Shop kombucha labelled non-alcoholic: halal. In the US it must stay below 0.5 percent alcohol.
- Hard kombucha: brewed to a higher alcohol level, often sold beside beer. This is haram.
- Home brewed kombucha: halal while it is fresh, but a long or warm second fermentation can raise the alcohol. Do not leave it to ferment for weeks, and throw away a batch that smells or tastes like alcohol.
Some unrefrigerated bottles keep fermenting on the shelf, which is why stores keep kombucha cold. Choose a trusted brand and store it in the fridge. For similar questions see non-alcoholic beer and soy sauce.
The evidence
Quran, Surah Al-Maaida 5:90
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِنَّمَا ٱلْخَمْرُ وَٱلْمَيْسِرُ وَٱلْأَنصَابُ وَٱلْأَزْلَٰمُ رِجْسٌۭ مِّنْ عَمَلِ ٱلشَّيْطَٰنِ فَٱجْتَنِبُوهُ لَعَلَّكُمْ تُفْلِحُونَ ﴿٩٠﴾
O you who have believed, indeed, intoxicants, gambling, [sacrificing on] stone alters [to other than Allah], and divining arrows are but defilement from the work of Satan, so avoid it that you may be successful.
Forbids what intoxicates, not every fermented food.
Sahih Muslim 2004
Sahih... فإن بقي شيء سقاه الخادم أو أمر به فصب ...
"He would drink the soaked drink for up to three days, and if any was left after that, he gave it to a servant or had it poured away."
A drink that has started to ferment is lawful until it can intoxicate.
Sunan Abi Dawud 3681
Graded hasan sahih... ما أسكر كثيره فقليله حرام ...
"Whatever intoxicates in a large amount, a small amount of it is forbidden."
Ordinary kombucha cannot intoxicate in any amount, while hard kombucha can.
Where the ruling is recorded
- al-Hidaya, Kitab al-Ashriba. Soaked and fermented drinks are lawful until they intoxicate.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Ashriba. Rulings on nabidh and fermented drinks.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.