Floor Space Index = (Total Area of all Floors of an Apartment Complex*) / (Total area of the Plot belonging to the Apartment Complex)
This means that if the total Plot area is 10,000 sqft., and FSI is 1:2, the Apartment Complex can have Total Floor Area (or Built Up Area) of 20,000 sqft. This in turn means that the Apartment Complex will have multiple Floors. Higher the FSI of an area, taller the Apartment Complex can grow.
* – Certain areas such as Balconies, FlowerBeds etc. may be excluded from the Floor area The exclusions depend on the exact FSI definition available in each City/Municipalty.
The FSI of each location of a City is decided by the Municipal Corporation of the City. It varies depending on the infrastructure of the area. If an area has robust water supply, sewage networks, public parks, transportation and other infrastructure it may have higher FSI.