Attenuation Calculator

About this calculator

Attenuation measures how much of the sugar in wort a yeast strain actually fermented into alcohol, calculated from the original gravity (OG) taken before fermentation and the final gravity (FG) taken after. Apparent attenuation — the simple version, (OG − FG) / OG × 100 — is what most homebrewers check first, but it overstates the real number because dissolved alcohol reads lighter on a hydrometer than the sugar it replaced.

Real attenuation corrects for that alcohol effect using a version of the Balling formula that weights the OG and FG readings in degrees Plato, giving a more accurate picture of how much fermentable sugar was actually consumed. Brewers compare their real attenuation against a yeast strain’s published range — commonly around 73–77% for many ale strains — to judge whether fermentation finished healthily or stalled early, and to fine-tune mash temperature or pitch rate on the next batch.

This calculator takes your OG and FG readings and returns both the apparent and real attenuation percentages.