Apparent attenuation is calculated directly from a pair of OG and FG readings — (OG - FG) / (OG - 1) x 100 — with no correction applied for the fact that dissolved alcohol itself reads lower on a hydrometer than the sugar it replaced. It's the number nearly every brewer means when they casually say "attenuation," and the figure printed on most yeast packaging and manufacturer data sheets.
Why "apparent" is in the name at all
The word "apparent" is doing real work here — because alcohol is less dense than water, a hydrometer reads a partly-fermented wort's gravity as lower than its true remaining sugar content would suggest on its own, meaning apparent attenuation slightly overstates how much sugar actually fermented out compared to real attenuation's corrected figure. For everyday homebrewing purposes the distinction rarely matters; for understanding exactly why a yeast strain's published range and your own calculated number might not line up precisely, it does.
Where the number actually comes from on this site
Every ABV calculation on this site reports apparent attenuation automatically alongside real attenuation — run your own OG and FG through the ABV calculator to see both figures side by side rather than calculating apparent attenuation by hand. Comparing your result against a strain's published range is the fastest sanity check on whether a fermentation finished where it should have.