Floor numbering is the numbering scheme used for a building's floors; it varies depending on the level of the "first floor" and on the names given to the subterranean levels.

Rooms numbers generally start with the floor's number; occasionally the first element may be the letter representing the floor. In large buildings, two conventions are common:

• Odd numbers are used for one side of the building and even for the other.
• The second digit in the room number indicates a specific block or wing of the building.

Lower floors

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Subterranean levels vary in numbering systems, often using B for Basement or P for Parking, for the first level below ground, although L (or LL) for Lower Level or Lobby is sometimes used. The next level down may be SB for "Sub-Basement", although numbering more frequently occurs at this point, either B1/P1 etc. or use of negative numbers -1 etc.

It is worth noting that in certain buildings in the US with a subterranean parking lot, the basement parking floor is labelled G, representing "garage". Such a labelling in elevators may be misleading for foreigners, as the button for G is directly below the button for the first floor and may be understood as "ground".

First floor

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In British English, in reference to typical buildings, the "first floor" is the first floor above the ground; but in American English, it is another name for the ground floor.

Most European countries, countries of the Commonwealth (except Singapore and Canada, which use the American system), and former British colonies like Hong Kong, follow the same convention as the British. Russia, some countries of East Europe, and most of eastern Asia (including China and Japan) have a convention similar to the American system. Hong Kong is unusual in that it follows the British rule in English, but for some old tenament buildings the Chinese characters follow the American rule.

Higher floors are then numbered consecutively in each case, as illustrated by the following table:

British convention	American convention	Hong Kong convention	Japanese convention
3rd floor	4th floor	3rd floor (and 三樓, 四樓 (literally 4th floor) for old tenant buildings)	4F
2nd floor	3rd floor	2nd floor (and 二樓, 三樓 (literally 3rd floor) for old tenant buildings)	3F
1st floor	2nd floor	1st floor (and 一樓, 二樓 (literally 2nd floor) for old tenant buildings)	2F
Ground floor	Ground or 1st floor	Ground floor and 地下 (literally Ground floor)	1F or 地上階 (Ground floor)

It is obvious that this can lead to some confusion, but little else can be done other than being aware of this issue. Put simply:

• American English floor number minus 1 = British English name
• British English floor number plus 1 = American English name

In North America, some buildings may have entrances on two different floors, such as those built into a hill. In these cases, the ground floor is the lower and the first floor is the upper. In the UK, the lower of these floors would be known as the lower ground floor, while the upper would be called either the upper ground floor or simply the ground floor. Also, some U.S. high-rise buildings follow the British system, often out of a desire on the part of the building's architect or owners to suggest a posh U.K./European setting.

Idiosyncrasies

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Unusual numbering exists in some hotels, for example, the uppermost level may be PH (for Penthouse), R (for Roof), or O (for Observation Deck), and the entrance level may sometimes be denoted M (for Main), or L (for Lobby). However, some buildings use extremely idiosyncratic denotation - one hotel in Toronto marks the first six floors as A, M, MM, C, H, and 1 (for Arcade, Main, Main Mezzanine, Convention, Health Club, and 1st floor). Also, the North Carolina Museum of Art, whose entrance is on the third floor up, has the floors lettered C, B, A, (main floor) and O, meaning office.

It is not uncommon for American buildings to omit the number 13 in their floor numbering because of common superstition surrounding this number. The floor numbering may either go straight from 12 to 14, or the floor may be named something like 'Skyline' instead of numbered. See Thirteenth floor for more details. Further, because the number four sounds like "death" in Chinese and related languages, it is sometimes skipped in hospital buildings, and some business buildings follow this convention.

Occasionally, around the world, one finds buildings which deviate from the norm. Examples of such deviations include sometimes numbering basement or the lowest level basement as floor 1. Thus, the ground floor may be numbered 2 or even something higher e.g. 6. Another occasional deviation is numbering the ground floor as 0. One sometimes finds buildings where different parts of the same building are numbered differently, e.g. the carparks have a separate level numbering from the occupied spaces at the same level. Often these buildings will have a star next to the elevator button for the floor with the "main" street level exit.

Many shopping centres in Hong Kong use unusual floor labelling systems. For instance, Festival Walk, a high-class mall in Kowloon Tong, has effectively four "ground floors" labelled LG2 (lower ground 2), LG1, G, and UG (upper ground). "Level 1" at Festival Walk would actually be the fourth floor if LG2 were to be considered the ground floor.

Romania uses Roman numerals for floor numbers.

Floor zero

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In buildings in the USA, there is no floor numbered zero. The first floor is immediately above the first basement.

At the Toronto Eaton Centre, in Canada, an additional shopping floor had been excavated underneath an existing multi-floor department store in the 1980's. It is designated Level Zero.

In Europe the ground level is the 0. Sometimes, it has more than one name: e.g., in the same building in Italy on one elevator the ground level is called "0", on others "PT" (piano terra = "ground level"). But in nearly every building the floor above the ground level is "1".

Trivia

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• The office building featured in the movie Being John Malkovich has a 7½th floor.
• William Tenn's short story The Tenants (1954) deals with renting a non-existent thirteenth floor.

Buildings and structures | Architectural elements | Identifiers