This advanced calculator gives you the full fermentation picture from just two hydrometer readings. Beyond ABV, it reports alcohol by weight, apparent and real attenuation, and the original, apparent, and real extract in degrees Plato — the numbers brewers use to judge yeast performance and recipe efficiency.
How it works
Each gravity reading is first converted to degrees Plato (a percentage of dissolved solids by weight) using the standard cubic polynomial:
P = −616.868 + 1111.14·SG − 630.272·SG² + 135.997·SG³
From the original extract OE (from OG) and apparent extract AE (from FG) the
tool derives:
- ABV with the advanced formula
ABV = (76.08 × (OG − FG) ÷ (1.775 − OG)) × (FG ÷ 0.794) - ABW (alcohol by weight) ≈
ABV × 0.79336 ÷ FG - Real extract
RE = 0.1808 × OE + 0.8192 × AE - Apparent attenuation
(OE − AE) ÷ OE × 100 - Real attenuation
(OE − RE) ÷ OE × 100
Why real vs apparent matters
A hydrometer can’t tell sugar from alcohol — it only measures density. Because ethanol is lighter than water, your FG reads lower than the true residual sugar, which inflates apparent attenuation. Real attenuation removes that distortion, so it is the figure to use when comparing yeast strains or diagnosing whether a batch finished as designed.
Example and notes
An amber ale at OG 1.060 finishing at FG 1.012 lands around 6.4% ABV with roughly 80% apparent attenuation and about 65% real attenuation. If your real attenuation comes out far below the yeast manufacturer’s stated range, suspect underpitching, low fermentation temperature, or unfermentable wort. Take your FG only after gravity has held steady for several days.