Fermentation Basics
How to Measure Alcohol Content at Home
Learn what hydrometer readings can tell you about fermentation strength, and where common homebrew estimates fall short.
Take a reliable starting reading
A hydrometer estimates liquid density relative to water. Measure original gravity (OG) in a representative, well-mixed sample before fermentation begins; bubbles, suspended pulp, or temperature differences can distort a reading.
Check the instrument’s calibration temperature and use its correction guidance when the sample is warmer or cooler. Record the value, temperature, date, and any recipe additions.
Measure after fermentation
When fermentation appears complete, take a sanitized sample and record final gravity (FG). Repeat the reading after several days; a stable result is a more useful endpoint than an airlock that has stopped bubbling.
A common estimate is ABV (%) ≈ (OG − FG) × 131.25. It is an approximation, especially for high-gravity, sweet, distilled, or otherwise unusual products. A laboratory method is needed when precise alcohol content matters.
Use instruments for their intended purpose
A refractometer is convenient before fermentation, but alcohol affects its reading afterward. Apply a validated correction using the original reading, or use a hydrometer for final gravity.
Never infer alcohol strength from taste, bubbles, or a single uncorrected instrument reading. Follow local laws and use only food-grade ingredients and equipment.