You know that the kanji refers to a floor (as in the "fifth floor"). This word represents the version that comes after a number, shown with a 〜. Essentially, this has the same meaning as the kanji you learned, making it nice and easy.
The reading is the same as the one you learned with the kanji. Don't type in the 〜 or you'll get it wrong!
"Which floor?" "Fourth please." "This elevator doesn't stop on that floor."
Koichi lives on the top floor of a ninety-nine story apartment building.
I live on the thirteenth floor of this apartment.