Attenuation is the share of a wort's fermentable extract that yeast actually converts into alcohol and carbon dioxide during fermentation, expressed as a percentage. It's one of the most quoted numbers on any yeast strain's product data sheet, and one of the most commonly misunderstood, because there are two different ways to calculate it from the same pair of gravity readings.
Apparent vs. real attenuation
Apparent attenuation is calculated straight from your OG and FG readings — (OG - FG) / (OG - 1) x 100 — and it's what most brewers mean day to day when they casually say "attenuation." It's also the number printed on most yeast packaging and manufacturer data sheets.
Real attenuation corrects for the fact that dissolved alcohol makes a hydrometer read artificially low — a wort that's already partly converted to alcohol shows a lower gravity than its true remaining sugar content would suggest on its own, because ethanol is less dense than water. Real attenuation is always a few percentage points lower than apparent attenuation for the same OG/FG pair, and it's the more scientifically accurate figure for how much sugar actually remains unfermented.
Why it matters
A yeast strain's published attenuation range (see the yeast reference hub for real strain data) tells you roughly how dry a finish to expect before you've even pitched it — a strain rated 65-70% will leave a noticeably sweeter, fuller-bodied beer than one rated 78-82%, everything else held equal. Actual attenuation in your own fermenter also depends on mash temperature (a lower mash temperature produces more fermentable wort, pushing attenuation higher regardless of strain), pitch rate, and fermentation temperature.
The ABV calculator computes both apparent and real attenuation automatically from your OG and FG, alongside ABV by both the simple and advanced formulas.