Calculate alcohol by volume from gravity readings.
The standard ABV formula is ABV = (OG - FG) × 131.25. Original Gravity (OG) is measured before fermentation and indicates total sugar content. Final Gravity (FG) is measured after fermentation — the difference represents sugar converted to alcohol. Typical beer OG ranges from 1.030 (light lager) to 1.120+ (imperial stout). Apparent attenuation measures what percentage of sugar was fermented, typically 65-85% for most yeast strains. A hydrometer or refractometer measures gravity; refractometer readings after fermentation need correction since alcohol changes the refractive index.