A floor level (also called "floor", "story", or "storey") is a horizontal open and occupiable space inside a building, divided from other such spaces by horizontal plates. The number of floors is determined by counting all levels from the lowest to the highest.

Floor levels may be classified as above-ground or below-ground. Each floor level must be at least 2 meters high to count as a standard floor. If a building is set on sloping ground, then the first level flush with or higher than the lowest immediately surrounding ground level is the first above-ground floor. Floors which cross the ground level by more than a meter in each direction are considered half-floors on either side (half over ground, half under ground). Floors below the base of the building are considered underground, even if they face a sunken plaza or moat.

Interior levels which are inaccessible to tenants and users, such as mechanical floors and hollow spaces under decorative domes, are counted as regular floors unless they are set back from the highest usable floor at the top of the building. If such spaces are set back at the bottom or middle levels, they are still counted. If a building's roof is accessible to the tenants, then even a set-back penthouse is counted as a floor if it holds any common space beyond a stairway landing.

If a building has any very narrow, neck-like feature at its bottom, which does not hold any usable space, then any stairway landings arranged in the same pattern as the building's other floors may be counted as floors. If a narrow tower rises at the top of a building, then its counted floor levels extend up to the highest level which holds any potential tenant or user space.

A mezzanine (any level whose floorplate does not extend fully to the building's perimeter) counts as a floor as long as both it and the floor below allow at least 2 meters of vertical room, AND as long as its area is large enough for reasonably flexible use (i.e. is not merely a balcony, pulpit, or landing).

In case the floors in a multi-story building occupy such a gradient of levels that it is impossible to count them by the above rules (e.g. if the floorplates are uneven or form a spiral) then the floors should be counted using the cross-section which provides the maximum number of floors.