Floor space index (FSI) value indicates the total amount of area (on all floors) which we can build upon a land plot.
FSI means the ratio between the area of a covered floor (Built up Area) to the area of that plot (land) on which a building needs to be constructed.
The floor space index (FSI) is governed by the development control (DC) rules of a city; it may also differ within a city depending upon zoning of city.
Please refer Development control rules of your area or get connected to an architect firm , they will be in a position to help up with prevailing FSI..