The floor area ratio
or floor space index
) is the ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
of the total floor area of buildings on a certain location to the size of the land of that location, or the limit imposed on such a ratio.
As a formula: Floor area ratio = (Total covered area on all floors of all buildings on a certain plot)/(Area of the plot)
Thus, an FSI of 2.0 would indicate that the total floor area of a building is two times the gross area of the plot on which it is constructed, as would be found in a multiple-story building.
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
other terms are used for the same measure, such as "floor space ratio" (FSR) in New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
and "Plot ratio" in Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
the terms "floor space index" (FSI) and "floor area ratio" (FAR) are both used.. .
In the United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
the terms "Plot ratio" and "Site ratio" are used.
Use in zoning
Floor area ratios are used as a measure of the intensity of the site being developed. It represents the mathematical formula of dividing the building area (measured in square feet) by 43,560 (number of square feet in an acre) to generate a ratio (expressed in a percentage) of building space: land area.
The floor area ratio can be used in zoning
Zoning is a device of land use planning used by local governments in most developed countries. The word is derived from the practice of designating permitted uses of land based on mapped zones which separate one set of land uses from another...
to limit the amount of construction in a certain area. For example, if the relevant zoning ordinance permits construction on a parcel, and if construction must adhere to a 0.10 FAR, then the total area of all floors in all buildings constructed on the parcel must be no more than one-tenth the area of the parcel itself.
An architect can plan for either a single-story building consuming the entire allowable area in one floor, or a multi-story building that rises higher above the plane of the land, but which must consequently result in a smaller footprint than would a single-story building of the same total floor area. By combining the horizontal and vertical limits into a single figure, some flexibility is permitted in building design
Building design refers to the broadly based architectural, engineering and technical applications to the design of buildings. All building projects require the services of a building designer, typically a licensed architect or structural engineer...
, while achieving a hard limit on at least one measure of overall size. One advantage to fixing this parameter, as opposed to others such as height, width, or length, is that floor area correlates well with other considerations relevant to zoning regulation, such as total parking that would be required for an office building
An office is generally a room or other area in which people work, but may also denote a position within an organization with specific duties attached to it ; the latter is in fact an earlier usage, office as place originally referring to the location of one's duty. When used as an adjective, the...
, total number of units that might be available for residential use, total load on municipal services
Municipal services or city services refer to basic services that residents of a city expect the city government to provide in exchange for the taxes which citizens pay. Basic city services may include sanitation , water, streets, schools, food inspection fire department, police, ambulance, and...
, etc. The amounts of these things tend to be constant for a given total floor area, regardless of how that area is distributed horizontally and vertically. Thus, many jurisdictions have found it unnecessary to include hard height limitations when using floor area ratio calculations.
Impact on land value
Edward Ptacek, 2009, noted that the allowable FAR has a major impact on the value of the land. Higher allowable FAR yields higher land value.
Andres Duany, 2000, notes 1) abdicating to floor area ratios (market forces) is the opposite of aiming a community toward something more than the sum of its parts. 2) FAR, a poor predictor of physical form, should not be used when the objective is to conserve and enhance neighborhood character. Whereas traditional design standards (height, lot coverage and setbacks or build-to lines) enable anyone to make reasonably accurate predictions, recognize violations, and feel secure in their investment decisions. And lastly, 3) if FAR is carelessly combined with traditional setbacks, assembled lots have a considerable advantage over individual lots, which has a negative effect on fine grained cities and the diversity of ownership.
Japan has extensively adopted the floor area ratio in the zoning system since 1970. The evaluation of the adoption is, however, controversial: somesay that it has deteriorated the skylines and building lines in Japanese cities; others claim that it has protected the residential environments.