This article is about the floor of a room or building. In mathematics, see floor function. In finance, see interest rate floor

In architecture, a floor is generally the lower horizontal surface of a room, but specially employed for one covered with boarding or parquetry (see also flooring). The various levels of rooms in a building are also called floors or stories, as "ground floor", "first story", "mezzanine floor", etc.

A confusion arises between American English and British English: In British English the floor at the ground level is the ground floor, and the floor above is the first floor, whereas in American English, the floor at the ground level is the first floor and the floor above is the second floor. However in either country, the first story is the floor at ground level.

The principal floor is the storey which contains the chief apartments whether on the ground floor or the floor above; in Italy they are always on the latter and known as the piano nobile. The storey below the ground floor is called the basement floor, even if only a little below the level of the pavement outside; the story in a roof is known as the attic floor. The expressions one pair, two pair, etc., apply to the storeys above the first flight of stairs from the ground (see also carpentry).

See also: loudspeaker floor standing system.