also: なんかい
what floor; which floor
Used to ask which floor of a building. Both readings なんがい and なんかい are common; なんがい is slightly more formal.
Excuse me, what floor is this building?
Which floor is the meeting room on?
What floor is the bookstore on?
What floor does Tanaka live on?
How many floors does this apartment building have?
Standard kanji spelling. The reading なんがい is more common in careful speech; なんかい is also widely used.
Common colloquial reading. Both なんがい and なんかい are acceptable, but なんがい is often preferred in formal contexts.
Compound of 何 (nan, 'what') + 階 (kai/gai, 'floor'). The reading なんがい results from rendaku (sequential voicing) of かい to がい after なん.