Hard Kombucha ABV Calculator

Hard kombucha breaks the assumptions behind both instruments more than any other drink on this site. The base kombucha already contains acetic, gluconic and other organic acids from the SCOBY, plus a small amount of ethanol from the primary ferment. Acids raise both the refractometer Brix and the hydrometer gravity without being fermentable, so the OG you read after adding sugar for the secondary ferment overstates the sugar that will actually become alcohol. The refractometer mode corrects only the ethanol effect, not the acid offset, so treat its result as an upper bound.

For a number you would put on a label, or need for a legal threshold, neither method is adequate on its own. Use the hydrometer mode with temperature correction to track the fermentation, then verify the finished product with a laboratory method or a distillation-based test. The carbonation calculator handles the bottle-conditioning step that follows.

The full Terrill correction and the hydrometer temperature polynomial, with their references, are on the general fermentation ABV calculator. For the reasoning behind temperature correction see hydrometer temperature correction.

Worked example. After adding sugar to a finished base kombucha, the gravity reads 1.060, and the secondary ferment with a champagne yeast stops at 1.006. Balling gives 76.08 × 0.054 ÷ 0.715 = 5.75 percent by weight, and 5.75 × 1.006 ÷ 0.794 = 7.3 percent ABV. Apparent attenuation is 0.054 ÷ 0.060 = 90 percent. If about 0.005 of that original gravity was acid rather than sugar, the true fermentable-basis ABV is closer to 6.7 percent, which is the direction of error to keep in mind for any legal limit.

Kombucha is not a yeast-only fermentation. The culture pairs yeast with acetic acid bacteria, and those bacteria oxidise ethanol into acetic acid. Alcohol is being produced and consumed at the same time, so ABV can plateau or even fall while fermentation is still visibly active.

That competition makes timing matter more than it does for beer or wine. Hard kombucha is usually pushed toward alcohol by limiting bacterial activity, using a higher sugar charge, and pitching a dedicated alcohol-tolerant yeast rather than relying on the culture alone.

The rising acidity also complicates measurement. Dissolved acids contribute to density and to refractive index, so both hydrometer and refractometer readings drift as the batch sours, independently of any change in sugar or alcohol.

Typical range: original gravity 1.030 to 1.050 from sweet tea, giving a modest base ABV that is often raised with additional sugar in a second fermentation.

Apparent attenuation is less clean for hard kombucha than for beer or wine, because part of the measured original gravity is acid rather than sugar. A batch that reads 1.060 after the sugar addition may only hold the fermentable equivalent of 1.055, so the attenuation figure comes out a few points lower than the yeast actually achieved.

With a champagne or ale yeast pitched for the secondary ferment, expect the corrected attenuation to land between 85 and 95 percent and the final gravity between 1.004 and 1.010. The acids and the residual unfermentable solids keep it from dropping below 1.000 the way a dry cider does. If the gravity stops above 1.015, the usual cause is the low pH of the base kombucha inhibiting the yeast: many strains struggle below pH 3.0. Adjust pH up slightly on the next batch or choose a more acid-tolerant strain, rather than adding more sugar to an already stalled ferment.