Hard Kombucha ABV Calculator

A refractometer measures how much light bends when passing through a liquid. Before fermentation this works perfectly for measuring sugar concentration. After fermentation, alcohol in the wash bends light differently to water — making the reading appear lower than the actual gravity. If you use the raw refractometer reading as your FG you will overestimate your ABV.

This calculator applies the Terrill correction formula, which uses both the original Brix reading and the post-fermentation apparent Brix to recover the true final gravity:

Where Ri = initial Brix (OG reading) and Rf = apparent final Brix (post-fermentation refractometer reading). This is the most widely accepted correction formula for homebrewing and home distilling.

Most hydrometers are calibrated at 20°C (68°F). If your sample is warmer, the liquid is less dense and the hydrometer floats higher, giving a reading that is lower than the true gravity. The correction runs about +0.001 SG per 5°C just above calibration temperature and grows with the gap, reaching roughly +0.006 SG at 40°C. This calculator applies the polynomial correction formula for accurate results across a wide temperature range.

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 tells you what percentage of available sugars have fermented. Standard kombucha (first ferment) rarely produces meaningful alcohol on its own; hard kombucha adds sugar and a wine or champagne yeast in a second stage specifically to push ABV up.

Apparent attenuation is a less dependable guide for kombucha than for other ferments, because acetic acid bacteria consume some of the ethanol the yeast produce and rising acidity shifts the gravity reading itself. Treat the number as a rough progress indicator and confirm final strength by another method where it matters.