Apparent Attenuation

Attenuation asks what share of the wort's extract the yeast has consumed, and it is the single most useful number a hydrometer produces during fermentation. A wort at 1.050 that finishes at 1.010 has dropped 40 of its 50 points, so $40/50 = 80\%$ apparent attenuation. The denominator is the points above water — $OG - 1$ — and not the gravity itself, which is the first place people go wrong.

The word "apparent" is load-bearing. The hydrometer is reading density, and the beer now contains ethanol, which is about 79% as dense as water. So the final gravity is depressed by the alcohol as well as by the missing sugar, and the calculation credits the yeast with eating more than it did. REAL attenuation — the honest figure, obtained by boiling the alcohol off and restoring the volume with water, or by calculation from both readings — runs about 0.81 times the apparent figure. A beer at 80% apparent has genuinely fermented about 65% of its extract. Both numbers are used, both are legitimate, and mixing them up when comparing a yeast strain's published attenuation range against your own reading is a common and quiet error.

What attenuation actually depends on is a three-way argument between the yeast, the mash and the wort. Yeast strains differ genuinely: many English ale strains stop around 70% because they cannot ferment maltotriose, while most lager strains and the saison strains push past 80% or 90%. But the mash sets the ceiling. Hold a rest at 63 °C and beta-amylase produces a highly fermentable, maltose-rich wort; hold it at 70 °C and alpha-amylase dominates, leaving long dextrins that no brewing yeast can touch. A brewer who wants a fuller beer changes the mash temperature, not the yeast.

Attenuation above 100% is not an error message. It means the final gravity fell below 1.000, which happens routinely in very dry beers, ciders and meads where there is enough alcohol and little enough residual extract that the liquid is genuinely lighter than water. The other end is the one to worry about: a reading stuck well below the strain's range, unchanged over three days, is a stalled fermentation. Rouse the yeast, raise the temperature a couple of degrees, and check the original pitch rate before reaching for more yeast — under-pitching is far more often the cause than a bad packet.