Apparent Attenuation Calculator

About this calculator

Apparent attenuation is the brewing-world measure of how much sugar yeast has converted into alcohol and CO₂ during fermentation, expressed as a percentage drop in specific gravity: AA% = (OG − FG) / (OG − 1) × 100, where OG is your original gravity reading before fermentation and FG is the final gravity once it settles. A beer that starts at 1.050 and finishes at 1.010, for example, shows about 80% apparent attenuation.

Homebrewers and professional brewers check this number against the published attenuation range for their yeast strain (Wyeast and White Labs list typical ranges like 73-77% for many ale strains) to judge whether fermentation performed as expected or stalled early — a sign of underpitching, low temperature, or a stressed yeast culture. It’s called “apparent” rather than “real” attenuation because a hydrometer reading is skewed by the alcohol already present in the beer, which reads lighter than it truly is; true attenuation, used mainly in lab settings, corrects for that effect.

Enter your original and final gravity readings and this calculator returns the apparent attenuation percentage, so you can quickly compare your batch against your yeast’s expected performance.