Calculate the alcohol percentage (ABV) of your homebrew, based on gravity measurements before and after fermentation.
OG (Original Gravity) and FG (Final Gravity) are density measurements of the wort before and after fermentation, taken with a hydrometer or refractometer. The difference between them tells you how much sugar the yeast has converted to alcohol:
The more sugar converted to alcohol, the bigger the difference between OG and FG, and the higher the alcohol percentage.
OG (Original Gravity) is the density of the wort before fermentation, and FG (Final Gravity) is the density after fermentation is complete. Both are usually measured with a hydrometer or refractometer.
The formula gives a good estimate for most homebrews, but is simplified. Actual alcohol percentage can vary somewhat depending on yeast type, fermentation temperature, and how accurately the measurements were taken.
Use a hydrometer (lowered into the liquid) or a refractometer (a few drops on the lens) to read the density. Measure OG right before adding yeast, and FG once fermentation has stabilized over several days.